From The Art and Popular Culture Encyclopedia
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.
As a euphemism
"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.