Odds are the kid is already banging away anyhoo, so what's the harm?
When I was 16 I managed to get myself into a variety of 18+ places. Buy alcohol. Go to Casino's, Clubs, Pubs, Skimpys and so on. I don't really see this as a big deal, especially if the kid is already sexually active and open about things of these nature. I had a stripper at my 18th, and I was no more mature at age 18 than 16. My experiences are relevant too, because I was 18 but a few years back, unlike many people who post here.