Flatbush, there is a big difference between 16 and 21. You really don’t see that?
Having recently been a teenager(I’m 26), I remember how much many of the guys in my neighborhood loved Purim – it was one of the few days they could get drunk with “their parents permission.” Don’t fool yourself into thinking that these boys are necessarily doing it l’shaim mitzvah.
This is a better question to broach with a Rabbi, but with the dangers associated with drinking (at any age), if you arent sure your child is doing it l’shaim mitzvah, its probably better to keep them sober.
If you dont mind them drinking, make sure they do it AT HOME where they are fully supervised and give them dry red wine. Most teenagers do not enjoy that. See if its l’shaim mitzvah or l’shaim alcohol…