For premature aging disorders, see Progeroid syndromes.

Senescence (from , meaning "to grow old," from ) or biological aging (also spelled biological ageing) is the gradual deterioration of function characteristic of most complex lifeforms, arguably found in all biological kingdoms, that on the level of the organism increases mortality after maturation. The word "senescence" can refer either to cellular senescence or to senescence of the whole organism. It is commonly believed that cellular senescence underlies organismal senescence. The science of biological aging is biogerontology.