Ageing is the progressive biological process associated with increasing age and involving gradual changes in cellular, tissue and whole-organism function. It is distinct from the simple passage of chronological time because individuals of the same age can exhibit substantially different physiological states. Biological ageing involves interacting processes including genomic instability, accumulation of cellular damage, changes in protein maintenance, mitochondrial dysfunction, altered nutrient sensing, cellular senescence, stem-cell exhaustion, changes in intercellular communication and chronic low-grade inflammatory activity.
These processes do not operate independently and can reinforce one another. Ageing is also influenced by inherited genetic variation, environmental exposures, nutrition, physical activity, infections and lifelong disease burden. The term ageing therefore encompasses both the measurable passage of time and the biological changes associated with it. Research aims to understand why physiological resilience changes with age and why different tissues and individuals age at different rates.
Cellular senescence is an important component of Ageing. Senescent cells have generally undergone a durable reduction or cessation of proliferation while remaining metabolically active. They can develop a senescence-associated secretory phenotype involving cytokines, chemokines, growth factors and proteases that alter their surrounding environment. Senescence can be beneficial when it prevents damaged cells from proliferating or participates in wound healing and development, but accumulation of senescent cells with age may contribute to tissue dysfunction.
Programmed cell death and immune clearance normally remove many damaged or senescent cells, but these processes can become less efficient or overwhelmed. The distinction between temporary stress responses and stable cellular senescence is important because not every non-dividing cell is senescent. Research into senolytic strategies seeks to remove selected senescent cells, although the safety, specificity and clinical effectiveness of such approaches remain active areas of investigation.
Genomic maintenance is another major component of ageing biology. Cells experience DNA damage from endogenous metabolic processes and environmental sources, and multiple repair systems continuously detect and correct lesions. Telomeres, repetitive DNA structures at chromosome ends, generally shorten during repeated cell division in many somatic cell types, although telomere dynamics vary among tissues and organisms. Mitochondria also undergo changes with age, including alterations in mitochondrial DNA, energy metabolism and quality-control processes.
Autophagy and related mechanisms remove damaged cellular components and recycle their molecular constituents, but their efficiency can change with age. Protein homeostasis, or proteostasis, involves synthesis, folding, trafficking and degradation of proteins, and disruption can allow damaged or misfolded proteins to accumulate. These processes are interconnected, meaning that impaired quality control can increase cellular stress, which can further damage DNA, proteins and organelles.
Ageing affects organs differently and does not occur at a uniform rate throughout the body. The cardiovascular system can undergo changes in arterial stiffness, vascular function and cardiac reserve. The skeletal system experiences changes in bone remodelling and muscle mass, while the nervous system can show alterations in processing speed, sensory function and some aspects of memory. The immune system undergoes immunosenescence, involving changes in immune-cell populations, responsiveness and inflammatory regulation.
The kidneys, lungs, endocrine organs and other tissues also undergo age-associated changes. These changes reduce physiological reserve, meaning that an older individual may recover more slowly from physiological stress even when resting function remains adequate. Importantly, ageing should not be equated with inevitable severe disease. Many older people retain high functional capacity, demonstrating that health outcomes reflect interactions among biological ageing, disease prevention, genetics, environment and lifelong behaviour.
Evolutionary theories of ageing attempt to explain why natural selection does not eliminate all age-associated deterioration. One influential concept is mutation accumulation, in which deleterious effects that occur mainly after reproduction may be weakly selected against. Another is antagonistic pleiotropy, where a genetic effect may provide an early-life reproductive advantage while producing harmful consequences later. The disposable soma concept proposes that organisms face trade-offs in allocating resources between reproduction and long-term somatic maintenance.
These theories address evolutionary explanations rather than providing a single molecular mechanism. They can coexist with mechanistic models involving DNA repair, cellular senescence, mitochondrial maintenance and nutrient sensing. Comparative biology shows that ageing patterns vary greatly among species, indicating that lifespan and age-related deterioration are evolutionarily modifiable traits rather than fixed properties of all complex organisms.
Nutrition, physical activity, sleep, smoking, alcohol exposure, infection history and environmental conditions can influence health trajectories during ageing. Regular physical activity can help preserve muscle strength, cardiovascular capacity and functional independence, while adequate nutrition supports tissue maintenance and immune function. Avoidance of tobacco is strongly associated with lower risk of numerous age-related diseases. However, lifestyle factors do not eliminate biological ageing, and individual outcomes remain influenced by genetics and previous exposures.
Preventive medicine becomes particularly important with increasing age because multiple chronic conditions can interact and because early detection may preserve function. Geriatric medicine often focuses on functional status, frailty, medication burden, cognition and multiple interacting conditions rather than treating each disease in isolation. This reflects the fact that ageing changes the relationship between physiological reserve, disease and recovery.
Ageing research increasingly distinguishes chronological age from biological or functional measures. Biomarkers of ageing may include molecular patterns, epigenetic changes, inflammatory markers, physiological measurements and composite indices, but no single biomarker perfectly captures biological age. Epigenetic clocks use patterns of DNA methylation to estimate age-related biological variation, while other approaches examine proteomic, metabolomic or transcriptomic changes. Such measurements are useful research tools but should not automatically be interpreted as precise measures of an individual's future lifespan or disease risk.
Longevity interventions are being investigated in model organisms and human studies, including approaches affecting nutrient-sensing pathways, senescent cells and metabolic regulation. Evidence from laboratory organisms does not automatically translate into demonstrated effects in humans. Ageing science therefore requires careful distinction between mechanistic plausibility, laboratory findings, observational associations and established clinical outcomes.
Ageing has broad significance because it influences disease patterns, healthcare systems, population structure and social organisation. Increased life expectancy has resulted from improvements in sanitation, nutrition, vaccination, medical treatment and living conditions, while population ageing creates new challenges involving chronic disease, disability and care provision. Healthy ageing focuses on maintaining functional ability and quality of life rather than merely extending chronological lifespan.
A common misconception is that ageing is caused by one universal process or that all age-associated changes are irreversible. In reality, different tissues show different trajectories, some age-related changes can be modified, and substantial variation exists among individuals. Another misconception is that chronological age alone determines biological capability. Ageing is better understood as a multidimensional process involving cumulative molecular and cellular changes, altered physiological resilience and interactions between biological inheritance and lifelong environmental conditions.
