Some people say that PI doesn't get too rowdy before 9 and that - that the humor is pretty PG - especially early - at the comedy club and AC - where you are probably most interested in going. But by 9:00, you will certainly see at least one young woman dressed appropriately for a nightclub, likely at least one display of PDA that will be appropriate for a nightclub (but not a themepark), and quite possibly someone who started early getting trashed.
Personally, I wish Disney would grow a backbone on this one and make PI off limits to anyone under 21. (Actually, I do think that parents bringing teenagers or older (17 and up) teenagers being alone there is fine, but it would be easier to card at the booth). I just think that adults should have places to go at Disney that is kid free - and a nightclub island seems like a reasonable choice for one of those places. But Disney hasn't, so you are free to do as you wish.