What is Ageing?
[[File:Population pyramid of Ireland 2016.png|thumb|250px|Population pyramid of Ireland 2016 showing ageing in one society]] '''Ageing''' or '''aging''' is the many changes that happen in an individual over time. In living things, ageing includes most physical and psychological changes which happen after adulthood. '''Senescence''' is the biological process which leads to ageing.<ref>Medawar P.B. & J.S. 1983. ''Aristotle to Zoos: a philosophical dictionary of biology''. Harvard University Press, p5{{ndash}}8. {{ISBN|0-674-04537-8}}</ref> There have been many attempts to answer the question: why do animals (especially humans) age?<ref>Comfort, Alex 1956. ''Ageing: the biology of senescence''. London: Routledge & Kegan Paul.</ref> In one way, ageing begins at [[birth]], because [[cell division]] and [[growth]] is fastest then, and gradually slows down as time goes on. In another way, ageing starts when the likelihood of [[death]] increases. [[Actuary|Actuarial tables]] show the likelihood of death at each stage of life. These tables are used by [[insurance]] companies to assess the rates of insurance on life policies and [[pension]]s. It turns out that we are least likely to die when we are adolescents and young adults. The reason suggested by biologists (such as [[Peter Medawar]] and [[George C. Williams]]) is that this is the time most significant for [[reproduction]], or was in mankind's past.<ref>Williams G.C. 1957. Pleiotropy, natural selection and senescence. ''Evolution'' '''11''', 398{{ndash}}411.</ref> The assumption is that the peak age of reproduction in mankind's history was lower than today.<ref>Medawar P.B. 1952. ''An unsolved problem of biology''. An inaugural lecture delivered at [[University College London]], 6 December, 1951. London: H.K. Lewis.</ref> A [[gene]] can be [[gene expression|expressed]] at various stages of life. Any [[allele]] of a gene which interfered with reproduction would have less chance of passing on to the next generation. The number of such genes in the population would a