Ageing (British English) or aging (American English) is the process of becoming older. It represents the accumulation of changes in a person over time. Ageing in humans refers to a multidimensional process of physical, psychological, and social change. Some dimensions of ageing grow and expand over time, while others decline. Reaction time, for example, may slow with age, while knowledge of world events and wisdom may expand. Research shows that even late in life, potential exists for physical, mental, and social growth and development. Ageing is an important part of all human societies reflecting the biological changes that occur, but also reflecting cultural and societal conventions. Ageing is among the largest known risk factors for most human diseases. Roughly 100,000 people worldwide die each day of age-related causes.Age is measured chronologically, and a person's birthday is often an important event. However the term "ageing" is somewhat ambiguous. Distinctions may be made between "universal ageing" (age changes that all people share) and "probabilistic ageing" (age changes that may happen to some, but not all people as they grow older including diseases such as type two diabetes). Chronological ageing may also be distinguished from "social ageing" (cultural age-expectations of how people should act as they grow older) and "biological ageing" (an organism's physical state as it ages). There is also a distinction between "proximal ageing" (age-based effects that come about because of factors in the recent past) and "distal ageing" (age-based differences that can be traced back to a cause early in person's life, such as childhood poliomyelitis). Chronological age does not correlate perfectly with functional age, i.e. two people may be of the same age, but differ in their mental and physical capacities. Each nation, government and non-government organisation has different ways of classifying age.Population ageing is the increase in the number and proportion of older people in society. Population ageing has three possible causes: migration, longer life expectancy (decreased death rate) and decreased birth rate. Ageing has a significant impact on society. Young people tend to commit most crimes, they are more likely to push for political and social change, to develop and adopt new technologies and to need education, the latter of which tend to lose political significance for people in the ageing process. Older people have different requirements from society and government as opposed to young people and frequently differing values as well, such as for property and pension rights. Older people are also far more likely to vote and in many countries the young are forbidden from voting. Thus, the aged have comparatively more, or at least different, political influence.Recent scientific successes in rejuvenation and extending a lifespan of model animals (mice 2.5 times, yeast and nematodes 10 times) and discovery of variety of species (including humans of advanced ages) having negligible senescence give hope to achieve negligible senescence (cancel ageing) for younger humans, reverse ageing or at least significantly delay it.However, human ageing may differ in significant respects from the ageing of worms and even mice. Recent progress in deciphering the ageing of human cells aged in culture as first described by Leonard Hayflick led to the demonstration that human cells age largely because of a genetic "clock" in the DNA region known as the telomere. By isolating the telomerase gene, scientists then at Geron demonstrated that telomerase was an immortalizing enzyme likely necessary for the immortality of germ-line cells, but absent in most cells in the body. This observation led to the mainstreaming of the concept of regenerative medicine, in which human embryonic stem cells may potentially be used to repair aged tissues with young cells, and induced pluripotent stem (iPS) cell technology capable of making cells of various kinds that are potentially useful in repairing tissues for the treatment of age-related degenerative diseases.Some aspects of bacterial senescence may lend support to contemporary theories of ageing, including the free radical, antagonistic pleiotropy, and disposable soma theories. Ageing is the major cause of mortality in the developed world.