oh, i dunno - a group of hot young women in a bar getting leathered, then all going off clubbing until 3am? i think i'd assume they were old enough to shag, if that was what i was after, tbh, even if not legally old enough to drink.
it's really gross when there is an obvious age difference - i'd wonder about the bloke's motives anyway. 'jail bait' has always been used to warn off those not old enough to be discerning - those who are only after a quick fumble in any case, because it's friday. etc, and are used to girls/ women looking for the same, and offering it.
i think we're talking about two different things though - much older guys perving about much younger girls, as opposed to those of much the same age who happen to be on opposite sides of a legal boundary, despite the younger party making enormous efforts to appear 'legal'.
no idea how much effort a 19yo bloke with the horn makes to check id though. 
i know my 21yo friend didn't check his id, lol. but she was so mortified i don't think she dated for a year after that, and it was always someone she 'knew', never anyone she met in a bar lol.