Ageing or aging (see spelling differences) is the process of becoming older. The term refers especially to humans, many other animals, and fungi, whereas for example bacteria, perennial plants and some simple animals are potentially biologically immortal.^[1] In the broader sense, ageing can refer to single cells within an organism which have ceased dividing (cellular senescence) or to the population of a species (population ageing).^[2]

In humans, ageing represents the accumulation of changes in a human being over time^[3] and can encompass physical, psychological, and social changes. 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 uncertain; current theories are assigned to the damage concept, whereby the accumulation of damage (such as DNA oxidation) may cause biological systems to fail, or to the programmed ageing concept, whereby problems with the internal processes (epigenomic maintenance such as DNA methylation ^[4]) may cause ageing. Programmed ageing should not be confused with programmed cell death (apoptosis). Additionally, there can be other reasons, which can speed up the rate of ageing in organisms including human beings like