What Is Aging? Biology in Plain Language

A short answer

In biology, aging is the gradual build-up of changes in the body that lowers physical and mental capacity, raises the risk of disease and, in the end, leads to death. The World Health Organization (WHO) describes it as the result of the accumulation of a wide variety of molecular and cellular damage over time. It adds that these changes are neither linear nor consistent and are only loosely associated with a person's age in years. [1] This page is education only. It does not tell anyone what to take or do, and it makes no claim that any product slows aging.

Evidence labels used here follow how we label evidence in Specialty Topics.

Age in years is not the whole story

The WHO says there is no typical older person: some 80-year-olds have physical and mental capacities similar to many 30-year-olds, while others lose capacity much earlier. It also says part of the variation in older people's health is genetic, but most is due to physical and social environments and personal characteristics. [1] This is why researchers distinguish chronological age (years since birth) from biological age (an estimate of how the body is functioning); see biological age vs chronological age.

Why do we age? Several explanations, not one

No single theory is settled. Three ideas come up often.

• Damage and limited repair. An influential evolutionary view holds that aging is caused not by active gene programming but by evolved limits in the body's maintenance, which lets damage build up. The same paper explains that animals invest differently in growth, reproduction and survival, which helps explain why species have different lifespans. [2]
• Hazard and trade-off theories. Classical evolutionary theories (mutation accumulation, antagonistic pleiotropy and disposable soma) predict that more outside danger, such as predation, selects for shorter lifespans. Many studies fit that prediction, but some data from guppies, nematodes and computer models do not, so the picture is still debated. [3]
• Hallmarks of aging. Researchers list shared cellular features of aging, such as genomic instability, epigenetic alterations (changes in how genes are switched on and off), mitochondrial dysfunction and cellular senescence (cells that stop dividing). [4] This is a way of organizing findings, not proof of cause; see the hallmarks of aging.

Label: established that biological aging involves accumulating cellular and molecular change [1]; mixed / still debated on why it happens and which processes drive it.

Aging and disease

The WHO lists common conditions of older age, including hearing loss, cataracts, back and neck pain, osteoarthritis, chronic lung disease, diabetes, depression and dementia, and says people are more likely to have several conditions at once as they age. [1] Researchers describe aging as the primary risk factor for major human diseases such as cancer, diabetes, cardiovascular disease and neurodegenerative disease [5], and the "geroscience hypothesis" proposes that targeting basic aging processes might delay several of them together. Early proof-of-concept trials are still being designed to test that idea. [6] It is a hypothesis, not an established treatment approach.

Whether aging itself should be called a disease is argued. One review noted that there is no common definition of what aging is, what causes it or what to target, and concluded that aging should not be considered a disease. [7] The regulatory side is covered in is aging a disease?.

What research cannot yet say

• No proven way to slow human aging. Lifespan findings in animals are preclinical (cells or animals only) and do not show benefit in people. See aging in animals vs humans.
• No consensus on how to measure aging. Biomarkers of aging currently lack agreed standards for what makes one reliable. [8]
• Environment matters. The WHO attributes most of the variation in older people's health to environments and personal characteristics rather than to genes. [1]

What to do with this

For practical, everyday guidance about staying healthy as you age, see the Wellness guide to healthy aging. For your own health questions, talk with a clinician.