Senescence

Senescence (/sɪˈnɛsəns/) (from Latin: senescere, meaning "to grow old," from senex) 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.

Senescence is not the inevitable fate of all organisms and can be delayed. The discovery, in 1934, that calorie restriction can extend lifespan twofold in rats, and the existence of species having negligible senescence and potentially immortal species such as Hydra, have motivated research into delaying and preventing senescence and thus age-related diseases. Organisms of some taxonomic groups (taxa), including some animals, even experience chronological decrease in mortality, for all or part of their life cycle. On the other extreme are accelerated aging diseases, rare in humans. There is also the extremely rare and poorly understood "Syndrome X," whereby a person remains physically and mentally an infant or child throughout one's life.