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Age is a measure of the duration of existence. In humans, it is most commonly expressed as chronological age, the time since birth, usually counted in years and, for infants, months. Chronological age guides many practical considerations such as education, health care, and eligibility for rights and responsibilities. Objects and biological samples also have ages, determined by methods appropriate to their nature (for example, radiocarbon dating for artifacts or geological dating for rocks).

Beyond chronological age, other concepts describe how old a being or thing is. Biological age refers to physiological condition relative to typical values for a given chronological age, influenced by genetics, lifestyle, disease, and environment. Epigenetic clocks estimate biological age from DNA methylation patterns. Social age concerns expected roles and statuses within a culture. Legal age thresholds define when activities are permitted or restricted, such as voting, drinking, driving, and marriage.

Age structure shapes populations. Demographers analyze the distribution of ages to project fertility, mortality, and dependency. In many parts of the world, populations are aging as birth rates fall and life expectancy rises, with implications for health systems, pensions, and the workforce. Policies commonly aim to support healthy aging and to address age-related inequalities.

Age also carries cultural and philosophical meanings. Attitudes toward aging vary, from reverence for elders to emphasis on youth. The concept intersects with disability, health, and social equity, influencing how societies allocate resources and opportunities. As science deepens our understanding of aging processes, measurements of age and the social meanings attached to it continue to evolve.