I really hate that this word (and others like it) have been corrupted in modern usage. This word is often understood to mean very nearly the opposite of what it ought to mean and what I generally use it to mean. Some euphemisms are good and helpful, but this is one that should be avoided. (See sense 1c.)
1. a. Grown up, having reached the age of maturity. (Of men, and, in mod. use, of animals.)
b. Of persons: characteristically mature in attitude, outlook, etc.; also, befitting or suitable for adults, as opp. to children or youngsters. Cf. GROWN-UP ppl. a. 2.
c. Applied euphem. to premises or productions ostensibly restricted to adult access, as adult cinema, entertainment, movie, etc.; pornographic, sexually explicit. N. Amer.
d. Of accommodation: designed for the use of elderly persons. Hence also, of or pertaining to the elderly. N. Amer.
2. fig. Of anything growing, as a plant, a language: Matured, full-grown.
1. a. A person adult; one who has reached maturity. adult baptism: the baptism of those only who are ecclesiastically adults, or of the years of discretion; opposed to infant baptism.
b. adult illiteracy, illiterate, literacy; adult education, the further education of those over ordinary school age (as in the universities), but commonly used of that provided by local educational authorities, etc.; adult training centre, a training centre at which (esp. young mentally handicapped) adults learn practical and other skills.