Ageing or aging is any change in an organism over time. Aging refers to a multidimensional process of physical, psychological, and social change (Hultsch and Deutsch). Some dimensions of aging 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 (Schaie). Research shows that even late in life potential exists for physical, mental, and social growth and development. This article focuses on the social, cognitive, cultural, and economic effects of ageing. The biology of ageing is treated in detail in senescence. Ageing is an important part of all human societies reflecting the biological changes that occur, but also reflecting cultural and societal conventions. Age is usually measured in full years — and months for young childrenoun A person's birthday is often an important event.