Adult

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mid 16th century: from Latin adultus, past participle of adolescere ‘grow to maturity’ (see adolescent).

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From French adulte, Latin adultus(“grown up”), perfect passive participle of adolescō(“I grow up”). Compare adolescent.

etymonline

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adult (adj.)

1530s (but not common until mid-17c.) "grown, mature," from Latin adultus "grown up, mature, adult, ripe," past participle of adolescere "grow up, come to maturity, ripen," from ad "to" (see ad-) + alescere "be nourished," hence, "increase, grow up," inchoative of alere "to nourish," from a suffixed form of PIE root *al- (2) "to grow, nourish."

Meaning "mature in attitude or outlook" is from 1929. As a euphemism for "pornographic," it dates to 1958 and does no honor to the word. In the old British film-rating system, A indicated "suitable for exhibit to adult audiences," and thus, implicitly, unsuitable for children (1914).

adult (n.)

"adult person," 1650s, from adult (adj.).