1) The age at which a girl is just old enough to have sex without getting the older man in trouble with the law.
2) A very fine pornographic magazine depicting teenage girls.
1) Jimmy: Hey Johnny, how old is your girlfriend? She looks really young.
Johnny: She just turned 18!
Jimmy: You lucky dog, she's barely legal!
2) I looked at the naked pictures in the latest issue of Barely Legal.
1) A person is only just above (by a matter of days to months) the legal age of consent.
2) A person featured in pronography is only just above (by a matter of days to months) the minimum age to legally appear in pornography.
3) A pornographic magazine in the USA. Also, a movie, TV series, album, movie etc.
1) "How old was that <dude/chick>? 18 and barely legal!"
2) "Hot barely legal <boys/girls> online now!"
3) "Barely Legal sux. I prefer the ladies."