Ageing is the progressive biological process associated with increasing age that involves changes in molecular, cellular, tissue, organ and whole-organism function. It is not synonymous with a particular disease, although ageing is the major risk factor for many chronic diseases and substantially alters susceptibility to injury and illness. Biological ageing involves accumulated alterations in DNA, proteins, cellular structures, metabolic systems, signalling pathways and tissue architecture. These changes occur alongside alterations in physiological reserve, repair capacity and responses to environmental stress.
Chronological age measures elapsed time since birth, whereas biological age refers more generally to the functional or molecular state of an individual and can differ substantially between people of the same chronological age. Ageing is therefore heterogeneous. Some individuals retain high levels of physical and cognitive function at advanced ages, whereas others experience substantial impairment earlier. Gerontology is the broad scientific study of ageing, while geriatrics is the medical specialty concerned with health and disease in older adults.
Modern biological theories of ageing emphasise multiple interacting mechanisms rather than a single universal cause. Important processes include genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem-cell exhaustion, altered intercellular communication, chronic inflammation and changes in extracellular matrix structure. These processes are interconnected.
For example, mitochondrial dysfunction can increase cellular stress, altered signalling can influence autophagy and metabolism, and persistent cellular senescence can change tissue environments through secretion of inflammatory and regulatory molecules. The widely used hallmarks-of-ageing framework groups major biological mechanisms into related categories, although the precise classification and causal importance of individual mechanisms remain active research questions. Ageing should therefore not be represented as a simple accumulation of one type of damage. It is a complex systems-level process involving altered maintenance, regulation, repair and adaptation across many biological scales.
Cellular senescence is one important component of ageing biology. A senescent cell has entered a state of durable cell-cycle arrest while remaining metabolically active. Senescence can arise following processes such as extensive cellular replication, DNA damage or oncogenic stress and can initially serve beneficial functions by preventing damaged cells from proliferating.
However, accumulation of senescent cells with age can become problematic because many senescent cells produce a collection of signalling molecules, proteases and other factors known collectively as the senescence-associated secretory phenotype. These secretions can influence neighbouring cells and contribute to inflammatory and tissue-remodelling processes. Autophagy and proteostasis are also important because cells must identify, repair, recycle or remove damaged proteins and organelles. Declining efficiency in these maintenance systems can contribute to cellular dysfunction. The balance between damage, repair, replacement and removal therefore plays a major role in determining how tissues change with age.
Ageing affects organs differently and at different rates. The cardiovascular system can develop increased arterial stiffness and altered vascular function, while skeletal muscle commonly loses mass and strength through processes collectively associated with sarcopenia. Bone remodelling changes with age and can increase susceptibility to fractures. The kidneys generally experience reductions in functional reserve, although considerable individual variation exists.
The immune system undergoes age-associated changes often described using terms such as immunosenescence and inflammageing, involving altered immune-cell populations and persistent low-grade inflammatory signalling. Sensory systems can also change, including age-related reductions in visual or auditory function. The nervous system retains substantial plasticity but may experience changes in processing speed, sleep architecture and some forms of memory. These changes should not automatically be interpreted as pathological. Distinguishing normal age-associated change from disease is an important part of clinical assessment.
Ageing is strongly influenced by both inherited biology and environmental exposures accumulated across the life course. Genetic variants can influence longevity and susceptibility to age-related diseases, but most variation in human ageing does not arise from a single ageing gene. Nutrition, physical activity, smoking, alcohol exposure, infection, sleep, socioeconomic conditions, occupational exposures, pollution and healthcare access can influence health trajectories.
Epigenetic patterns can change with age and can also respond to environmental conditions, making them potentially useful as biomarkers of biological ageing. Lifelong accumulation of exposures means that later-life health cannot always be explained by conditions present only during old age. The life-course approach therefore treats ageing as a process beginning early in life rather than an isolated stage that starts at retirement. Early developmental conditions, adult behaviours and cumulative socioeconomic circumstances can all contribute to later-life differences in function and disease risk.
Evolutionary theories of ageing attempt to explain why natural selection generally does not produce organisms with indefinitely maintained biological function. The mutation accumulation theory proposed by Peter Medawar emphasises the reduced effectiveness of natural selection against harmful genetic effects that occur predominantly after reproduction. The antagonistic pleiotropy theory associated with George C. Williams proposes that some genetic characteristics can increase reproductive success early in life while producing harmful effects later.
The disposable soma theory, associated with Thomas Kirkwood, emphasises trade-offs between investment in reproduction and maintenance and repair of the organism. These theories address different evolutionary mechanisms and are not necessarily mutually exclusive. They also concern evolutionary explanations for why ageing occurs rather than directly identifying the proximate molecular mechanisms operating inside cells. Modern ageing research often integrates evolutionary biology with molecular and physiological approaches.
Ageing is associated with increased probability of many conditions, including cardiovascular disease, several cancers, neurodegenerative disorders, osteoporosis, diabetes and sensory impairment, but ageing itself is not equivalent to disease. This distinction matters because treating every age-associated change as pathology can produce unnecessary medicalisation, while attributing genuine disease solely to age can result in missed diagnoses. Functional status, frailty and physiological reserve provide additional ways of characterising ageing beyond disease counts.
Frailty describes increased vulnerability to relatively minor stressors resulting from reduced physiological reserve across multiple systems and can be assessed using different conceptual and measurement approaches. Functional ageing can therefore differ substantially from chronological ageing. A person of advanced chronological age may remain independent and physically active, while another person of younger chronological age may have significant functional limitations. Clinical care increasingly recognises this heterogeneity when assessing older adults.
Research into ageing includes attempts to identify interventions that influence mechanisms associated with biological ageing or extend healthy function. Experimental research has investigated pathways involving nutrient sensing, cellular senescence, autophagy, inflammation and mitochondrial function, while human research has examined factors such as physical activity, dietary patterns, sleep and management of chronic disease. Some interventions produce promising findings in experimental organisms but cannot automatically be assumed to produce equivalent effects in humans.
Biomarkers such as epigenetic clocks, telomere measures, inflammatory markers and physiological indices are being investigated as potential measures of biological ageing, but none provides a complete universal definition of an individual's ageing process. A central distinction is therefore between increasing lifespan and preserving healthspan. Ageing research seeks to understand why biological function changes with time and how those changes interact with disease, resilience and environmental circumstances. The field has implications for medicine, demography, social care, economics and the design of societies in which people increasingly survive to older ages.
