Adult
The term adult describes any mature organism, but normally it refers to a human: one that is no longer a child / minor and is now either a man or a woman. Adulthood can be defined in terms of biology, psychological adult development, law, personal character, or social status. These different aspects of adulthood are often inconsistent and contradictory. A person may be biologically an adult, and have adult behavioral characteristics 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 define adult character.
Coming of age is the event; passing a series of tests to demonstrate the child 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.
"Adult" also means "not considered suitable for children," in particular as a euphemism for being related to sexual behaviour, such as adult entertainment, adult video, adult magazine, adult bookstore. However, adult education simply means education for adults, not particularly sex education.

# Biological adulthood
Adulthood is generally understood as the time when physical maturation is complete.  One reaches their maximum height and secondary sex characteristics form such as body hair and facial hair, voice lowers in pitch (especially noticeable in men), and menses begin (women). Natural sleep patterns change in adulthood, as adults typically require less sleep than during adolescence. One thing people don't specify is what (psychologically) an adult is. A common theory is that adulthood is the real test of life, to experience the world from a first-person standpoint instead of through the parents. Then the adult can pass those experiences down to younger people and they can experience them when they grow up.

# Legal adulthood
Legally it 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, being a soldier, 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, 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 model or actor in pornography: 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 the film, a cinema being allowed to let the young person (under 18) enter
The legal definition of entering adulthood usually varies between ages 1521, 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 age of the Bar Mitzvah), for Jewish boys, for example, were 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. 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 the United States, the United Kingdom, and China, the legal adult age is 18, with some exceptions:
In Britain society expects people to be adults when they are only 16 even though they are not adults until they are 20.

# Personal characteristics
There are some qualities that symbolize adultness in most cultures. Not always is there a concordance between the qualities and the physical age of the person.
The adult character comprises:
- Self-control - restraint, emotional control.
- Stability - stable personality, strength.
- Independence - ability to self-regulate.
- Seriousness - ability to deal with life in a serious manner.
- Responsibility - accountability, commitment and reliability.
- Method/Tact - ability to think ahead and plan for the future, patience.
- Endurance - ability and willingness to cope with difficulties that present themselves.
- Experience - breadth of mind, understanding.
- Objectivity - perspective and realism.

# Social status
Adults, as a class, especially middle-age adults, enjoy an elevated status in society. This so-called Adult Privilege works in the same way as White Privilege by conferring often unspoken advantages, exemptions or immunities to members of the class; it also takes the form of adultism, which is a predisposition towards adults, inherently biased against children, youth, and all young people who aren't addressed or viewed as adults.  For example, while society fixates on the supposedly immoral and destructive behavior of youth, adults are not held accountable when they often display the same if not worse behavior in certain areas such as drug abuse, obesity and crime.  Ironically the status we afford adults for their maturity includes the privilege to act immaturely.
With such apparent double standards, some social critics have defined adulthood as an organization or institution that believes they should always have the right to command and be obeyed.