Age affects health because biological systems change over time, but chronological age is only one determinant of an individual's health. Ageing involves accumulated molecular and cellular changes that can gradually reduce physiological reserve and increase susceptibility to some diseases. WHO describes ageing as involving accumulation of diverse molecular and cellular damage, with progressive changes in physical and mental capacity and increasing risk of disease, while emphasising that these changes are neither perfectly linear nor identical between individuals. Two people of the same age can therefore have very different functional capacities, disease burdens and healthcare needs. Age should be considered a risk factor and context rather than a complete description of biological health.
Childhood and adolescence involve growth and development rather than simply an accumulation of disease risk. The brain, bones, immune system, reproductive system and other organs undergo substantial developmental changes. Nutrition, sleep, physical activity, vaccination, environmental exposure, psychosocial conditions and access to healthcare during these periods can influence health later in life. Some diseases are more common in childhood, whereas others become more likely after decades of cumulative exposure. Developmental stages also alter how symptoms are expressed and how medicines are dosed, meaning that healthcare recommendations cannot simply be scaled from adults to children.
During adulthood, many chronic diseases become more common because risk accumulates over time. Repeated exposure to tobacco smoke, air pollution, poor diet, inactivity, alcohol, occupational hazards or other risks can contribute to cumulative damage. Blood pressure and vascular risk may change with age, while metabolic and musculoskeletal changes can affect body composition and physical capacity. However, age does not make deterioration inevitable at a fixed rate. WHO notes that healthy behaviours, including balanced nutrition, physical activity and avoidance of tobacco, contribute to reducing noncommunicable disease risk and delaying care dependency.
Older age is associated with increased prevalence of several conditions, including hearing loss, cataracts, osteoarthritis, chronic respiratory disease, diabetes, depression and dementia. Multiple conditions may occur simultaneously, creating multimorbidity and making healthcare more complex. WHO also identifies geriatric syndromes such as frailty, falls, urinary incontinence and delirium, which often arise from several interacting factors rather than a single disease. The presence of one condition can affect management of another, so older-adult healthcare frequently requires attention to function, medicines, cognition, mobility, nutrition and social circumstances in addition to individual diagnoses.
Ageing can alter physiological reserve, meaning that an older person may recover more slowly from illness, surgery, injury, dehydration or other physiological stressors. This does not mean that older adults are inherently frail. WHO explicitly notes that there is no typical older person and that some people in their eighties have physical and mental capacities comparable with much younger adults. Functional reserve is influenced by physical activity, nutrition, chronic disease, social circumstances, genetics and lifelong exposures. Maintaining muscle strength, cardiovascular fitness, balance and independence can therefore be important aspects of healthy ageing.
Age also changes preventive healthcare priorities. Earlier life may emphasise vaccination, developmental assessment, reproductive health and prevention of risky behaviours, while later adulthood may involve greater attention to cardiovascular risk, cancer screening, bone health, hearing, vision, falls, medication review and functional capacity. These priorities vary according to national guidelines and individual risk. Age-based screening thresholds exist because disease prevalence and the balance between benefits and harms change with age, but an age threshold does not mean that everyone inside or outside a particular group has identical risk.
Biological ageing interacts strongly with environment and socioeconomic circumstances. WHO states that differences in older-age health are influenced substantially by physical and social environments, including homes, neighbourhoods and communities, as well as personal characteristics and lifelong experiences. Safe housing, accessible transport, opportunities for physical activity, social connection and access to healthcare can help older people maintain functional ability. Conversely, poverty, unsafe housing, isolation or environmental hazards can accelerate functional decline or make existing limitations more consequential. Age therefore modifies the effects of other determinants rather than acting independently.
The central concept in healthy ageing is not simply extending lifespan but extending the period during which a person can maintain meaningful function and wellbeing. WHO defines healthy ageing as developing and maintaining functional ability that enables wellbeing in older age. This means age-related health assessment should consider what a person can do, how independently they can function, their cognitive and emotional wellbeing, their social environment and their ability to adapt to changes. Age increases the probability of many health problems, but it does not establish an inevitable trajectory, and chronological age alone cannot accurately predict an individual's overall health.
