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, or social status.
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, 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).
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.
Not to be confused with maturity:
Maturity is a psychological term used to indicate how a person responds to the circumstances or environment in an appropriate manner. This response is generally learned rather than instinctive, and is not determined by one's age. Maturity also encompasses being aware of the correct time and place to behave and knowing when to act appropriately, according to the situation and the culture of the society one lives in.
If you are a legal adult or not define the laws of your state according to your age. Regarding this topic the question should be changed to "when do we know that we are mature (enough)?"