Ageing in biology, the combination of changes in an organism that appear to occur inevitably and irreversibly with the passage of time, eventually resulting in death. Such changes vary considerably in time and severity of occurrence among different species and from one individual organism to the next. Among humans, they include a decrease in tissue flexibility, loss of some nerve cells, hardening of the blood vessels, and general decrease in body tone. Biologists concerned with ageing may investigate such changes, or they may focus on bodily deficits and disabilities that accumulate with age but that appear to result more directly from disease, stress, or environmental trauma (see Alzheimer’s Disease). No scientific consensus exists as to the true nature of the ageing process.


Although research into biological ageing is not guided by any single universally accepted theory, genetic, cellular, and physiological studies have yielded various hypotheses. One of the most prominent genetic concepts, the so-called error theory, assumes that the deficits of ageing result from the accumulation of random genetic damage, or from small errors in the flow of genetic information (see Genetics). Such damage or errors would reduce or prevent proper cell function.

In cellular research, the best-known theory of ageing rests on the so-called Hayflick effect, named after the American microbiologist Leonard Hayflick. Hayflick found that certain human cells in a tissue culture are capable of only a limited number of cell divisions before they die. This finding may suggest that ageing is programmed into cells, and it could account for differences in the lifespan of different animal species, as well as differences in longevity between sexes within the same species. Among humans, for example, females typically outlive males by about eight years.

Physiological theories of ageing focus on organ systems and their interrelationships. One area of much current investigation, for example, is the immune system, which protects an organism from foreign cells. A characteristic of mammals is that the immune system gradually loses its capacity to fight off infections and other invaders as the organism ages. As a consequence, antibodies are produced that are unable to distinguish between “friendly”, or “self”, and “enemy”, or “non-self”, cells in the organism. Most experts now believe that ageing is not the result of a single mechanism but represents many phenomena working in concert.


The process of human ageing must also be considered in the context of complex and changing societies. The ways in which people age are not entirely fixed by biology; they are also affected by individual environmental and social circumstances. Consequently, ageing is increasingly being studied as a process that includes psychosocial and cultural components; in addition, the subject is being extended to include the entire course of life, rather than being restricted to the period of old age.

Knowledge gained through the work of social and behavioural scientists is helping to dispel some myths about the inevitability of the ageing process. For example, one myth is that intelligence peaks in adolescence and then deteriorates; another is that sexual activity begins an irreversible decline in the middle years, whereas sexual activity is now known to continue even into the very late years of life.

Because the ageing process is not unalterable, behavioural researchers are seeking ways in which it can be modified. For example, certain memory-aiding strategies have been found to help reverse the short-term memory loss experienced by some old people. Nursing-home arrangements to promote independent behaviour have been found to bring patients to better levels of functioning, including some patients once thought to be hopelessly impaired. Relatively simple, research-tested changes in food flavourings have been found to solve nutritional problems that result from age-related declines in the senses of taste and smell. Artificial aids are also being improved to meet the visual and hearing problems of later life.


Understanding the ageing process more fully will require the combined efforts of psychosocial and biomedical scientists. New research methods being brought to bear on the subject include clinical trials in biomedical research and statistical analysis of data from long-term studies by social scientists. The following topics are of particular interest: nutritional requirements of the aged, age-related changes in reactions to drugs, and senile dementia.

Some researchers are also asking longer-range questions about how social change—changes in smoking, exercise, and dietary habits; economic fluctuations; political shifts; medical advances; and new technologies—can affect the ageing process.