It's not just the people under 21 (well, 16 here) you can be excluding on an alcohol party. There are also people that don't drink or drink very little.
(To be fair, I only got weird looks and remarks once for not drinking alcohol and that was at a student bar where drinking seemed to be the norm. Normally it isn't too big a deal, but I can imagine some people - especially younger ones - feeling uncomfortable being the only one not drinking alcohol. Even more so when a group just gets a pitcher of beer.)
I usually do drink at these kinds of events, but when I don't I've never felt pressured to (at least not since college). Maybe it's different for people who sometimes drink vs. never drink.
Well.. and PJ did shove like $15 in the jukebox and demand I pick songs, and gave us money for the taxi ride home which also paid for a nice pizza back at the hotel.
(For me, that's not true. I don't mind alcohol, but I can't stand the taste of beer. I can't finish a zip of it, let alone a glass. I do sometimes have to explain that. Since I'm old enough to have a driver's license I sometimes have the nice excuse that I still need to drive. Works a bit better in that it requires less explanation.)
People are silly.