aging

Introduction
Biological theories of aging
- Genetic theories
- Nongenetic theories
  - Wear-and-tear theory
  - Cross-linking theory
  - Autoimmune theory
  - theory
  - Oxidative damage theory
  - Psychosociological theory
Natural history of aging
- Reproduction and aging
- Species differences in longevity and aging
- The inheritance of longevity
Senescence in mammals
- Changes in body composition, metabolism, and activity
- Changes in structural tissues
- Tissue cell loss and replacement
- Mammalian cell cultures
- Changes in tissue and cell morphology
Aging at the molecular and cellular levels
- Aging of genetic information systems
- Aging of the immune system
- Aging of neural and endocrine systems
Internal and external causes of aging
- External environmental agents
  - Ionizing radiation
  - Temperature
  - Physical wear of nonrenewable structures
  - Infectious disease and nutrition
- Internal environment: consequences of metabolism
Anti-aging and longevity research
- Antioxidants
- Calorie restriction and longevity
- Sirtuins
- Rapamycin
- Stem cells
- Genetics and life span

References & Edit History Quick Facts & Related Topics

Images & Videos

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aposematic coloration; tiger moth

squirrel; longevity among mammals

A chart compares the maximum life spans of various animals, using icons and color coding for mammals, birds, other vertebrates, and invertebrates. Lifespans range from the mayfly at less than a year to the Greenland shark at 500 years, with humans, elephants, whales, birds, and reptiles shown at various points along the timeline. Measurement intervals are marked in years, with a key at the bottom explaining the color categories.

A group of senior citizens sits in chairs, smiling and holding their hands together in a prayer pose during an exercise class.

At a Glance

A group of senior citizens sits in chairs, smiling and holding their hands together in a prayer pose during an exercise class.

Related Questions

primates — primates Primates are among the longest-lived groups of mammals.

aging

life process

Also known as: ageing

Written by

Leonard P. GuarenteAll

Fact-checked by

Britannica Editors

Last updated

Oct. 19, 2026 •History

aging, progressive physiological changes in an organism that lead to senescence, or a decline of biological functions and of the organism’s ability to adapt to metabolic stress. Aging takes place in a cell, an organ, or the total organism with the passage of time. It is a process that goes on over the entire adult life span of any living thing. Gerontology, the study of the aging process, is devoted to the understanding and control...

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