Adult

Adult is an individual who has attained the age of majority. Note that "adult" is a term with legal significance and also with social significance. The age of majority varies by state and even by context (e.g., a 19-year-old can vote but not drink alcohol in many states).

Source
National Research Council, Computer Science and Telecommunications Board, "Youth, Pornography, and the Internet," app. B (Dick Thornburgh & Herbert S. Lin eds. 2002).