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. There have been many attempts to answer the question: why do animals (especially humans) age?
In one way, ageing begins at birth, because cell division and growth is fastest then, and gradually slows down as time goes on.