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Ageing (British English) or aging (American English) is the process of becoming older. In the narrow sense, the term refers to biological ageing of human beings, animals and other organisms. In the broader sense, ageing can refer to single cells within an organism (cellular senescence) or to the population of a species (population ageing). In humans, ageing represents the accumulation of changes in a human being over time, encompassing physical, psychological, and social change. Reaction time, for example, may slow with age, while knowledge of world events and wisdom may expand. Ageing is among the greatest known risk factors for most human diseases:〔 of the roughly 150,000 people who die each day across the globe, about two thirds die from age-related causes. The causes of ageing are unknown; current theories are assigned to the damage concept, whereby the accumulation of externally induced damage (such as DNA point mutations) may cause biological systems to fail, or to the programmed ageing concept, whereby internal processes (such as DNA telomere shortening) may cause ageing. 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 ageing and thus age-related diseases. == Ageing versus immortality == Human beings and members of many other species necessarily experience ageing and mortality. In contrast, some species can be considered immortal: for example, bacteria fission to produce daughter cells, strawberry plants grow runners to produce clones of themselves, and animals in the genus ''Hydra'' have a regenerative ability with which they avoid dying of old age. Even within humans and other mortal species, there are arguably cells with the potential for immortality: cancer cells which have lost the ability to die such as the HeLa cell line, stem cells, and specifically germ cells (producing ova and spermatozoa). In artificial cloning, adult cells can be rejuvenated back to embryonic status and then used to grow a new tissue or animal without ageing. Normal human cells however die after about 50 cell divisions in laboratory culture (the Hayflick Limit, discovered by Leonard Hayflick in 1961). After a period of near perfect renewal (in humans, between 20 and 35 years of age), ageing is characterised by the declining ability to respond to stress, increasing homeostatic imbalance and the increased risk of disease. This currently irreversible series of changes inevitably ends in death. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ageing」の詳細全文を読む スポンサード リンク
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