Aging (life cycle)

    Aging is any change in an organism over time. Aging refers to a multidimensional process of physical, psychological, and social change (Hultsch and Deutsch). Some dimensions of aging grow and expand over time, while others decline. Reaction time, for example, may slow with age, while knowledge of world events and wisdom may expand (Schaie). Research shows that even late in life potential exists for physical, mental, and social growth and development. Aging is an important part of all human societies reflecting the biological changes that occur, but also reflecting cultural and societal conventions. Age is usually measured in full years — and months for young children. A person's birthday is often an important event.

The term "aging" is somewhat ambiguous. Stuart-Hamilton (1994) notes how distinctions may be made between "universal aging" (age changes that all people share) and "probabilistic aging" (age changes that may happen to some, but not all people as they grow older, such as the onset of Type Two diabetes). Chronological aging, referring to how old a person is, is arguably the most straightforward definition of aging and may be distinguished from "social aging" (society's expectations of how people should act as they grow older) and "biological aging" (an organism's physical state as it ages). Stuart-Hamilton also notes distinction between "proximal aging" (age-based effects that come about because of factors in the recent past) and "distal aging" (age-based differences that can be traced back to a cause early in person's life, such as childhood poliomyelitis).

Differences are sometimes made between populations of adults; as McFadden (2005) points out, divisions are sometimes made between the young old (65-74), the middle old (75-84) and the oldest old (those aged 85 and above). However, as McFadden notes, problematic in this is that chronological age does not correlate perfectly with functional age, i.e. two people may be of the same age, but differ in their mental and physical capacities.

Population ageing is the increase in the number and proportion of older people in society. Population aging has three possible causes: migration, longer life expectancy (decreased death rate), and decreased birth rate. The societal effects of age are great. Young people tend to commit most crimes, they are more likely to push for political and social change, to develop and adopt new technologies, and to need education. Older people have different requirements from society and government as opposed to young people, and frequently differing values as well. Older people are also far more likely to vote, and in many countries the young are forbidden from voting, and thus the aged have comparatively more political influence.

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Senescence

Main article: Senescence

In biology, senescence is the state or process of aging. Cellular senescence is a phenomenon where isolated cells demonstrate a limited ability to divide in culture (the Hayflick Limit, discovered by Leonard Hayflick in 1965), while Organismal senescence is the aging of organisms.

After a period of near perfect renewal (in Humans, between 20 and 50 years of age), organismal sencescence is characterized by the declining ability to respond to stress, increasing homeostatic imbalance and increased risk of disease. This irreversible series of changes inevitably ends in death.

Some researchers (specifically biogerontologists) are treating aging as a disease. As genes that have an effect on aging are discovered, aging is increasingly being regarded in a similar fashion to other genetic conditions, potentially "treatable."

Indeed, aging is not an unavoidable property of life. Instead, it is the result of a genetic program. Numerous species show no sign of aging, the best known being perennial plants (e.g. trees) which can live thousands of years and be multiplied by cuttings without limit. Most microbes and some animals, e.g. amphibians and large fish, also seem to be free of aging. In these species, adults constantly reproduce only to destroy their young, usually by eating them. Therefore, "immortal" species evolve more slowly than "mortal" species.

Aging is believed to be favoured by natural selection because it accelerates the evolution rate of a species by increasing the number of generations per unit of time. By dying away, the old individuals liberate the resources for their offsprings, thus increasing their chance of survival. Essentially, aging is the result of investing resources in reproduction rather than maintenance of the body, the "Disposable Soma" theory.

In humans and other animals, cellular senescence has been attributed to the shortening of telomeres with each cell cycle; when telomeres become too short, the cells die. The length of telomeres is therefore the "molecular clock" predicted by Hayflick. Telomere length is maintained in immortal cells (e.g. germ cells) by the enzyme telomerase. In the laboratory, mortal cell lines can be immortalized by the activation of their telomerase gene, present in all cells but active in few cell types. Cancerous cells must become immortal to multiply without limit. This important step towards carcinogenesis implies, in 85% of cancers, the reactivation of their telomerase gene by mutation. Since this mutation is rare, the telomere "clock" can be seen as a protective mechanism against cancer.

Other genes are known to affect the aging process, the sirtuin family of genes have been shown to have a significant effect on the lifespan of yeast and nematodes. Over-expression of the RAS2 gene increases lifespan in yeast substantially.

In addition to genetic ties to lifespan, diet has been shown to substantially affect lifespan in many animals. Specifically, caloric restriction (that is, restricting calories to 30-50% less than an ad libitum animal would consume, while still maintaining proper nutrient intake), has been shown to increase lifespan in mice up to 50%. Caloric restriction works on many other species beyond mice (including species as diverse as yeast and Drosophila), and appears (though the data is not conclusive) to increase lifespan in primates according to a study done on Rhesus monkeys at the National Institute of Health (US). Since, at the molecular level, age is counted not as time but as the number of cell doublings, this effect of calorie reduction could be mediated by the slowing of cellular growth and, therefore, the lengthening of the time between cell divisions.

Drug companies are currently searching for ways to mimic the lifespan-extending effects of caloric restriction without having to severely reduce food consumption.

Dividing the lifespan

A human life is often divided into various ages. Because biological changes are slow moving and vary from person to person, arbitrary dates are usually set to mark periods of human life. In some cultures the divisions given below are quite varied.

In the USA, adulthood legally begins at the age of eighteen or nineteen, while old age is considered to begin at the age of legal retirement (approximately 65).

• Pre-conception: Ovum, Spermatozoon, Pre-existence
• Conception: Fertilization
• Pre-birth: Conception to birth
• Infancy: Birth to 2
• Childhood: 2 to 12
• Adolescence: 13 to 20
• Early Adulthood: 21 to 34
• Middle Adulthood: 35 to 54
• Late Adulthood: 55+
• Death
• Post-Death: Decomposition, Cryonics, (Afterlife, Ghost)

Ages can also be divided by decade:

• Denarian: someone between 10 and 19 years of age
• Vicenarian: someone between 20 and 29 years of age
• Tricenarian: someone between 30 and 39 years of age
• Quadragenarian: someone between 40 and 49 years of age
• Quinquagenarian: someone between 50 and 59 years of age
• Sexagenarian: someone between 60 and 69 years of age
• Septuagenarian: someone between 70 and 79 years of age
• Octogenarian: someone between 80 and 89 years of age
• Nonagenarian: someone between 90 and 99 years of age
• Centenarian: someone between 100 and 109 years of age
• Supercentenarian: someone over 110 years of age

See also Seven ages of man for an older system of dividing the human life.

Cultural variations

In some cultures (for example Serbian and Russian) there are two ways to express age: by counting years with or without including current year. For example, it could be said about the same person that he is twenty years old or that he is in twenty-first year of his life.

Considerable numbers of cultures have less of a problem with age compared with what has been described above, and it is seen as an important status to reach stages in life, rather than defined numerical ages. Advanced age is given more respect and status.

Traditional Chinese culture uses a different aging method, called Xusui (虛歲) with respect to common aging which is called Zhousui (周歲). In the Xusui method, people are born at age 1, not age 0. See also East Asian age reckoning for more information.

Society

Legal

There are variations in many countries as to what age a person legally becomes an adult.

Most legal systems define a specific age for when an individual is allowed or obliged to do something. These ages include voting age, drinking age, age of consent, age of majority, age of criminal responsibility, marriageable age, age where one can hold public office, and mandatory retirement age. Admission to a movie for instance, may depend on age according to a motion picture rating system. A bus fare might be discounted for the young or old.

Similarly in many countries in jurisprudence, the defence of infancy is a form of defence by which a defendant argues that, at the time a law was broken, they were not liable for their actions, and thus should not be held liable for a crime. Many courts recognize that defendants, which are considered to be juveniles, may avoid criminal prosecution on account of their age.

Economics and marketing

The economics of aging are also of great import. Children and teenagers have little money of their own, but most of it is available for buying consumer goods. They also have considerable impact on how their parents spend money.

Young adults are an even more valuable cohort. They often have jobs with few responsibilities such as a mortgage or children. They do not yet have set buying habits and are more open to new products.

The young are thus the central target of marketers.^[1] Television is programmed to attract the range of 15 to 35 year olds. Movies are also built around appealing to the young.

Health care demand

Many societies in the rich world, i.e. Western Europe and Japan, have aging populations. While the effects on society are complex, there is a concern about the impact on health care demand. Saltman et al. (2006) argue that the large number of suggestions in the literature for specific interventions to cope with the expected increase in demand for long-term care in aging societies can be organized under four headings: improve system performance; redesign service delivery; support informal caregivers; and shift demographic parameters.

Cognitive effects

Steady decline in many cognitive processes are seen across the lifespan, starting in one's thirties. Research has focused in particular on memory and aging, and has found decline in many types of memory with aging, but not in semantic memory or general knowledge such as vocabulary definitions, which typically increases or remains steady. Changes in cognition with age are discussed by Stuart-Hamilton (1994). As Stuart-Hamilton notes, early studies generally found declines in intelligence in the elderly, but may be criticised for being cross-sectional studies rather than longitudinal studies. Interestingly, evidence suggests that verbal intelligence may show a less sharp decline than other forms of intelligence. Creativity may also show a decline in age. While it is popularly believed that as people age, after around the age of thirty, intellectual skill will show a gradual decline, a rather different theory discussed by Stuart-Hamilton (1994) is the "terminal drop theory", which suggests that intellectual skills remain steady throughout life, and then plummet sharply as people near the end of their lives. Individual variations in rate of cognitive decline may, according to this theory, be explained in terms of people have different lengths of life.

Coping with demands of later life

A considerable literature in psychology has examined coping in the elderly. Various factors, such as social support, religion and spirituality, active engagement with life and having an internal locus of control have been proposed as being beneficial in helping people to cope with stressful life events in later life. Indeed, social support and personal control have been described as being "perhaps the two most important predictors of morbidity, mortality and well-being in adulthood" (Smith, Kohm, Savage, Stevens, Finch Ingate & Lim, 2000; p458). Regarding locus of control, a classic study by Langer and Rodin (1976) found that in one wing of an old person's home, where people had control over their environment, people had better self-ratings of health, lower risk of mortality and were more alert than in another wing where they had less control. As Aldwin and Gilmer (2004) point out, there is evidence that external locus of control has a negative effect on a person's ability to cope, although Aldwin and Gilmer (2004) point out that this has not been replicated when a more specific health-related locus of control measure is taken. One specific measure of locus of control that has been found to be associated with coping in the elderly is social control, perceptions of how much influence one has over one's social relationships, and evidence suggests that this may act as a moderator variable for the relationship between social support and perceived health in the elderly (Bisconti & Bergeman, 1999). Aldwin and Gilmer (2004) propose that there are five types of coping that elderly may use: - Problem-focussed coping; - Emotion-focussed coping; - Social support; - Making meaning; - Religious coping.

Considerable literature contests the view that the outcomes of retirement are all negative for people. As Hayslip and Panek (1989) note, evidence suggests that retirement may have both positive and negative consequences.

Religion has been an important factor used by the elderly in coping with the demands of later life; as McFadden (2005) notes, "Beginning in the 1980s, evidence emerged that older adults spontaneously mention religious coping far more often than other forms of coping with major life stressors" (McFadden, 2005, p170). Some evidence also suggests that religious commitment may be associated with reduced mortality. However, it is important to appreciate that religion may be a multidimensional variable. There is evidence that while, with advancing years, participation in religious activities in a formal sense may decline, the elderly continue to practice religion in a more informal manner - this has led to the well-known distinction in religious gerontology between organizational religiosity (participation in formal religious activities, such as attendance at church services) and nonorganizational religiosity (use of more informal means to express religious commitments, such as prayer in the privacy of one's own home) (Mindel & Vaughan, 1978).

One variable that has been found to correlate positively with well-being in the elderly is Self-Rated Health (SRH). This has been linked to reduced mortality; as Idler (2003) points out, beginning in the 1980s, evidence emerged that elerdly adults who rated their own health as "excellent" had a lower risk of mortality than those who rated their health as "fair" or "poor". Various reasons may exist why this association exists - although it may seem an obvious finding insofar as people who rate their own health better may have better health objectively, as Benyamini, Blumstein, Lusky and Modan (2003) point out, this has been observed even in studies which have controlled for socioeconomic status, psychological functioning and health status. Sex-based differences may complicate findings here - as Deeg and Bath (2003) note, the effect appears to be stronger for men than for women.This is a general finding, but as Benyamini et al. (2003) point out, a few studies have found either no sex-based differences or else a stronger link between SRH and mortality for women than for men; also, as Benyamini point out, data are complicated here by the findings from some studies that suggest sex-based differences are only evident for certain age groups, for cerain causes of death of for certain levels of self-ratings of health Nevertheless, there is certainly evidence, from at least forty studies, that good SRH in the elderly is associated with increased well-being and reduced mortality and morbidity (Deeg, 2003). A study by Bowling (2005) suggests that when elderly people are asked to name the single most important factor that contributes to their quality of life, the two most frequently voiced answers are social relationships (which, in Bowling's view, can imply relationships with pets as well as with other human beings), and health. There are also articles suggesting the importance of locus of control to coping with aging (Heckhausen & Schulz, 1995; Windsor et al, 2007).

Societal impact

Societal aging refers to the demographic aging of populations and societies.^[2] Cultural differences in attitudes to aging have been studied.^[3]

Emotional improvement

Given the physical and cognitive declines seen in aging, a surprising finding is that emotional experience improves with age. Older adults are better at regulating their emotions and experience negative affect less frequently than younger adults and show a positivity effect in their attention and memory. The emotional improvements show up in longitudinal studies as well as in cross-sectional studies, and so cannot be entirely due to only the happier individuals surviving.

Terminology

The concept of successful aging can be traced back to the 1950s, and popularised in the 1980s. Previous research into aging exaggerated the extent to which health disabilities, such as diabetes or osteoporosis, could be attributed exclusively to age, and research in gerontology exaggerated the homogeneity of samples of elderly people.^[4][5]

Successful aging consists of three components:^[6]

1. Low probability of disease or disability;
2. High cognitive and physical function capacity;
3. Active engagement with life.

A greater number of people self-report successful aging than those that strictly meet these criteria.^[4]

Successful aging is viewed by Fentleman, Smith and Peterson(1990) as an interdisciplinary concept, spanning both psychology and sociology. They state that in the behavioural sciences, successful aging is to be understood as "a quality of the transaction between the changing person and the changing society over the entire life span, but especially during a person's later years" (Fentleman et alia, 1990; p50).

Healthy aging has been proposed as a more appropriate term.^[4]

Optimal aging better takes into account how many elderly people suffer some health detriments, the cultural diversity in approaches to death and how, in Western Europe and Northern America, people may approach death may differ from approaches taken in other cultures.^[7]

Vaillant (2002; cited in Aldwin & Gilmer, 2004) has listed six dimensions of optimal aging:

1. No physical disability over the age of 75 as rated by a physician; 2. Good subjective health assessment (i.e. good self-ratings of one's health); 3. Length of undisabled life; 4. Good mental health; 5. Objective social support; 6. Self-rated life satisfaction in eight domains, namely marriage, income-related work, children, friendship and social contacts, hobbies, community service activities, religion and recreation/ sports.

Theories

Non-biological theories

Modernization Theory

This is the view that the status of the elderly has declined since industrialization and the spread of technology.

Disengagement Theory

This is the idea that separation of older people from active roles in society is normal and appropriate, and benefits both society and older individuals. Disengagement theory, first proposed by Cumming and Henry (1961), as cited in Stuart-Hamilton (1994), has received considerable attention in gerontology, but has been much criticised. Schaie and Willis (1996) note that the original data on which Cumming and Henry based the theory were from a rather small sample of older adults in Kansas City, and from this select sample Cumming and Henry then took disengagement to be a universal theory. As Stuart-Hamilton (1994) notes, there are research data suggesting that the elderly who do become detached from society as those were initially reclusive individuals, and such disengagement is not purely a response to aging. In response to the many criticisms that have been levelled against disengagement theory, a very different theory has been proposed:

Activity Theory

In contrast to disengagement theroy, this theory implies that the more active elderly people are, the more likely they are to be satisfied with life. The view that elderly adults should maintain well-being by keepin active has had a considerable history, and since the work of Lemon, Bengston and Peterson (1972; cited in Schaie & Willis, 1996), this has become to be known as activity theory. However, as Stuart-Hamilton (1994) notes, this theory may be just as inappropriate as disengagement for some people. Stuart-Hamilton argues that current received wisdom on the psychology of aging is that both disengagement theory and activity theory may be optimal for certain people in old age, depending on both circumstances and personality traits of the individual concerned. There are also data which query whether, as activity theory implies, greater social activity is linked with well-being in adulthood (Schaie & Willis, 1996). One theory that has been proposed as a compromise between activity theory and disengagement theory is Cartensen's (1991) selectivity theory, suggesting that it may benefit older people to become more active in some aspects of their lives, more disengaged in others (Schaie & Willis, 1996).

Continuity Theory

The view that in aging people are inclined to maintain, as much as they can, the same habits, personalities, and styles of life that they have developed in earlier years. Continuity theory, as Bowling (2005) explains, is Atchley's theory that individuals, in later life, make adaptations to enable them to gain a sense of continuity between the past and the present, and the theory implies that this sense of continuity helps to contribute to well-being in later life. Bowling describes disengagement theory, activity theory and continuity theory as being "Social theories of ageing (sic.)" but adds that each has been criticised, stating that they are now perceived as "dated products of their era" (Bowling, 2005, p3). Bowling lists problems with these theories, such as whether they take enough cognizance of socio-political issues, and describes theories which, in her view, may be more promising.

Cognitive Theory

A view of aging that emphasizes individual subjective perception, rather than actual objective change itself, as the factor that determines behavior associated with advanced age.

Demographic Transition Theory

The idea that population aging can be explained by a decline in both birthrates and death rates following industrialization.

Disuse theory. The idea states that cognitive and physical skills will atrophy unless one continuously practices them.

Exchange Theory

The idea that interaction in social groups is based on the reciprocal balancing of rewards depending on actions performed

Political Economy Theory

A societal perspective on the aging process that explains that the status and resources of the elderly, as well as how people age, are shaped by each person's place in the social structure and the economic and political forces that impact their sociopolitical location.

• Selectivity Theory - see above for under Activity Theory, for more on how Cartensens' (1991) selectivity theory acts a mediator between disengagement theory and activy theory (Schaie & Willis, 1996)

Biological theories

Telomere Theory

Telomeres (structures at the ends of chromosomes) have been show experimentally to shorten with each successive cell division. Shortened telomeres activate mechanism that prevent further cell multiplication. This may be an important mechanism of aging in tissues like bone marrow and the arterial lining where active cell division is necessary.

Reproductive-Cell Cycle Theory

The idea that aging is regulated by reproductive hormones that act in an antagonistic pleiotrophic manner via cell cycle signaling, promoting growth and development early in life in order to achieve reproduction, but later in life, in a futile attempt to maintain reproduction, become dysregulated and drive senescence (dyosis).

Wear-and-Tear theory

The idea that changes associated with aging are the result of chance damage that accumulates over time.

Somatic Mutation Theory

This is the biological theory that aging results from damage to the genetic integrity of the body’s cells.

Error Accumulation Theory

This is the idea that aging results from chance events that gradually damage the genetic code.

Accumulative-Waste Theory

The biological theory of aging that points to a buildup of cells of waste products that presumably interferes with metabolism.

Autoimmune Theory

This is the idea that aging results from an increase in autoantibodies that attack the body's tissues. A number of diseases associated with aging, such as atrophic gastritis and Hashimoto's thyroiditis, are probably autoimmune in this way.

Aging-Clock Theory

The idea that aging results from a preprogrammed sequence, as in a clock, built into the operation of the nervous or endocrine system of the body. In rapidly dividing cells the shortening of the telomeres would provide just such a clock.

Cross-Linkage Theory

This is the idea that aging results from accumulation of cross-linked compounds that interfere with normal cell function.

Free-Radical Theory

The idea that free radicals (unstable and highly reactive organic molecules, also named reactive oxygen species or oxidative stress) create damage that gives rise to symptoms we recognize as aging.

Mitohormesis

It has been known since the 1930s that restricting calories while maintaining adequate amounts of other nutrients prevents aging across a broad range of organism. Recently, Michael Ristow has shown that this delay of aging is due to increased formation of free radicals within the mitochondria causing a secondary induction of increased antioxidant defence capacity.^[8]

Measure of age

The normal point of time from where to measure the age of a human being is from birth. However, this is not how gynaecologists measure age in prenatal development. Rather, age for people not yet born is normally measured in gestational age, taking the last menstruation of the woman as a point of beginning. Alternatively, fertilization age, beginning from fertilization can be taken.

See also

• Adultism
• Aging brain
• Aging Research Centre
• Biodemography
• Biological immortality
• Calorie restriction
• Life expectancy
• List of life extension-related topics
• Longevity
• Maturity (psychological)
• Memory and aging
• Michael Ristow
• Mitohormesis
• Orthomolecular medicine
• Population aging
• Retirement
• Senescence
• The Grim Reaper Gene
• Youth bulge

Notes

References

1. ^ Krulwich, Robert (2006). Does Age Quash Our Spirit of Adventure?. All Things Considered. NPR. Retrieved on 2006-08-22.
2. ^ Sarah Harper, 2006, Aging Societies: Myths, Challenges and Opportunities.
3. ^ Best and Williams; see cross-cultural psychology
4. ^ ^{a b c} Strawbridge et al. (2002)
5. ^ Rowe and Kahn (1987)
6. ^ Rowe and Kahn (1997)
7. ^ Aldwin and Gilmer (2004)
8. ^ Schulz TJ, Zarse K, Voigt A, Urban N, Birringer M, Ristow M (2007). "Glucose restriction extends Caenorhabditis elegans life span by inducing mitochondrial respiration and increasing oxidative stress". Cell Metab. 6 (4): 280–93. doi:10.1016/j.cmet.2007.08.011. PMID 17908557.

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