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."