Adult

An Adult is a person who has completed puberty, and is physically/sexually mature regardless of physical strength, muscle tone or age.

The word can have a sociolegal meaning:

1. Legal majority (usually age 18). Someone who has reached the age of majority and can legally vote in elections and referenda. In most instances this indicates people who have passed their 18th birthday, antonym of this use is minor.
2. In slang, various things people do are considered a passage into adulthood, like earning a paycheck, driving a car, living on your own, getting married, etc.