Biologically, an adult is a human being or other organism that has reached sexual maturity. In human context, the term adult additionally has meanings associated with social and legal concepts. In contrast to a "minor", a legal adult is a person who has attained the age of majority and is therefore regarded as independent, self-sufficient, and responsible.
Human adulthood encompasses psychological adult development. Definitions 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 an adult character.
In different cultures there are events that relate passing from being a child to becoming an adult or coming of age. This often encompasses the 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.
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.
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.
Puberty 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.
The legal definition of entering adulthood usually varies between ages 16–21, depending on the region in question. Some cultures in Africa[clarification needed] define adulthood at age 13.
According to Jewish tradition, adulthood is reached at age 13 (the minimal age of the Bar Mitzvah or Bat Mitzvah) for Jewish boys and girls; they are expected to demonstrate preparation for adulthood by learning the Torah and other Jewish practices. The Christian Bible and Jewish scripture contain no age requirement for adulthood or marrying, which includes engaging in sexual activity. 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 for most purposes, with some notable exceptions: