An adult is a human being or living organism that is of relatively mature age, typically associated with sexual maturity and the attainment of reproductive age. In human context, the term has other subordinate meanings associated with social and legal concepts; for example, a legal adult is a legal concept for a person who has attained the age of majority and is therefore regarded as independent, self-sufficient, and responsible (contrast with "minor"). Adulthood can be defined in terms of physiology, psychological adult development, law, personal character, emotional development, and/or social status.
These different aspects of adulthood are often inconsistent and contradictory. A person may be biologically an adult, and have adult behavior but still be treated as a child if they are under the legal age of majority. Conversely one may legally be an adult but possess none of the maturity and responsibility that may define adult character.
An event relating to the oncoming of adulthood is coming of age, which encompasses passing a series of tests to demonstrate that a person is prepared for adulthood, or reaching a specified age, sometimes in conjunction with demonstrating preparation. Most modern societies determine legal adulthood based on reaching a legally specified age without requiring a demonstration of physical maturity or preparation for adulthood.
Some propose that moving into adulthood involves an emotional structuring of denial, suggesting this process becomes necessary to cope with one's own behavior, especially in uncomfortable situations, and also the behavior of others.
Biological adulthood 
Historically and cross-culturally, adulthood has been determined primarily by the start of puberty (the appearance of secondary sex characteristics such as menstruation in women, ejaculation in men, and pubic hair in both sexes). In the past, a person usually moved from the status of child directly to the status of adult, often with this shift being marked by some type of coming-of-age test or ceremony.
Most medical and general English dictionaries define childhood as the period from infancy to puberty, thus historically adulthood began with puberty. After the social construct of adolescence was created, adulthood split into two forms: biological adulthood and social adulthood. Thus, there are now two primary forms of adults: biological adults (people who have attained reproductive ability, are fertile, or who evidence secondary sex characteristics) and social adults (people who are recognized by their culture and/or law as being adults). Depending on the context, adult can indicate either definition.
Although few or no established dictionaries provide a definition for the two word term biological adult, the first definition of adult in multiple dictionaries includes "the stage of the life cycle of an animal after reproductive capacity has been attained". Thus, the base definition of the word adult is the period beginning at puberty. Although this is the primary definition of the base word adult, the two word term biological adult stresses or clarifies that the original definition, based on the beginning of puberty, is being used (that is, the organism has matured to the biologically important point of being able to reproduce).
Puberty and therefore biological adulthood generally begins around 10 or 11 years of age for girls and 11 or 12 years of age for boys, though this will vary from person to person. Because the term adult is most often used without the adjective social or biological, and since the term is frequently used to refer to social adults, some writers have taken the meaning of the two word phrase biological adult to begin at the end of physical maturation rather than the onset of puberty.
Legal adulthood 
Legally, adulthood means that one can engage in a contract. The same or a different minimum age may be applicable to, for example, parents losing parenting rights and duties regarding the person concerned, parents losing financial responsibility, marriage, voting, having a job, serving in the military, buying/possessing firearms (if legal at all), driving, traveling abroad, involvement with alcoholic beverages (if legal at all), smoking, sex, gambling (both lottery and casino) being a prostitute or a client of a prostitute (if legal at all), being a model or actor in pornography, etc. Admission of a young person to a place may be restricted because of danger for that person, concern that the place may lead the person to immoral behavior, and/or because of the risk that the young person causes damage (for example, at an exhibition of fragile items).
One can distinguish the legality of acts of a young person, and of enabling a young person to carry out that act, by selling, renting out, showing, permitting entrance, participating, etc. There may be distinction between commercially and socially enabling. Sometimes there is the requirement of supervision by a legal guardian, or just by an adult. Sometimes there is no requirement, but just a recommendation.
With regard to pornography one can distinguish:
- being allowed inside an adult establishment
- being allowed to purchase pornography
- being allowed to possess pornography
- another person being allowed to sell, rent out, or show the young person pornography, see disseminating pornography to a minor
- being a pornographic actor: rules for the young person, and for other people, regarding production, possession, etc. (see child pornography)
With regard to films with violence, etc.:
- another person being allowed to sell, rent out, or show the young person a film, a cinema being allowed to let a young person (under 18) enter
The legal definition of entering adulthood usually varies between ages 16–21, depending on the region in question. Some cultures in Africa define adult at age 13.
According to Jewish tradition, adulthood is reached at age 13 (the minimal age of the Bar Mitzvah), for Jewish boys, for example, are expected to demonstrate preparation for adulthood by learning the Torah and other Jewish practices. The Christian Bible and Jewish scripture has no age requirement for adulthood or marrying, which includes engaging in sexual activity. The age of consent for sexual relations in the Vatican was previously 12, but the law stating this is not valid any more.
The 1983 Code of Canon Law states, "A man before he has completed his sixteenth year of age, and likewise a woman before she has completed her fourteenth year of age, cannot enter a valid marriage". According to The Disappearance of Childhood by Neil Postman, the Christian Church of the Middle Ages considered the age of accountability, when a person could be tried and even executed as an adult, to be age 7.
In most of the world, including most of the United States, parts of the United Kingdom (England, Northern Ireland, Wales), India and China, the legal adult age is 18 (historically 21) for most purposes, with some notable exceptions:
- The United Kingdom: Scotland (16)
- British Columbia, New Brunswick, Newfoundland and Labrador, Northwest Territories, Nova Scotia, Nunavut, Yukon Territory in Canada; Nebraska and Alabama in The United States, and South Korea (19)
- Indonesia and Japan (20)
See also 
- Maranz Henig, Robin (2010-08-18). "What Is It About 20-Somethings?". New York Times. p. 10. Retrieved 2010-09-24. "THE DISCOVERY OF adolescence is generally dated to 1904, with the publication of the massive study “Adolescence,” by G. Stanley Hall, a prominent psychologist and first president of the American Psychological Association."
- International Dictionary of Medicine and Biology (1986)
- Churchill’s Medical Dictionary (1989)
- (Chumlea, 1982).
- "For girls, puberty begins around 10 or 11 years of age and ends around age 16. Boys enter puberty later than girls-usually around 12 years of age-and it lasts until around age 16 or 17." "Teenage Growth & Development: 11 to 14 Years". pamf.org.
- canon 1083, §1