Aging out is American popular culture vernacular used to describe anytime a youth leaves a formal system of care designed to provide services below a certain age level.

There are a variety of applications of the phrase throughout the youth development field. In respect to foster care, aging out is the process of a youth transitioning from the formal control of the foster care system towards independent living. It is used to describe anytime a foster youth leaves the varying factors of foster care, including home, school and financial systems. The United States Citizenship and Immigration Services defines an "aging out" case as, "a situation referring to a person's petition to become a permanent legal resident as a child, and in the time that passes during the processing of the application, the child turns 21 and ages out.

Often used to highlight the problems traditional foster care approaches face, aging out affects foster youth in a variety of ways. An estimated 30,000 adolescents age out of the foster care system each year in the United States.

Aging (American English, occasionally British English) or ageing (British English) is 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.^{[citation needed]} Ageing is an important part of all human societies reflecting the biological changes that occur, but also reflecting cultural and societal conventions. 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).^{[citation needed]} 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).