What is Adult?

Legal Definition
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.
-- Wikipedia
Legal Definition
A person is considered a legal adult at the age of 18. In family law, the age a person may consent to enter into marriage may be less than 18 years, and will differ from state to state. Although 18 is the age a parent's child support obligation terminates, in some states it may be 21 years, primarily if the child is attending college.
Legal Definition
In the civil law. An infant who, if a boy, has attained his full age of fourteen years, and if a girl, her full age of twelve. Domat, Liv. Prel. t. 2, s. 2, n. 8. In the common law an adult is considered one of full age. 1 Swanst. R. 553.
-- Bouviers Law Dictionary
Legal Definition
In the civil law. A male infant who has attained the age of fourteen; a female infant who has attained the age of twelve. Dom. IAv. Prel. tit 2, ยง 2, n. 8. In the common law. One who has attained the legal age of majority, generally 21 years, though in some states women are legally "adults" at 18. Schenault v. State, 10 Tex. App. 410; George v. State, 11 Tex. App. 95; Wilson v. Lawrence, 70 Ark. 545, 69 S. W. 570.
-- Black's Law Dictionary