College kids still have access to alcohol before they're 21 through their friends. A lot more drinking happens at parties than at the pub. It's the combination of underage access and the lack of supervision (because at the pub, the staff can cut off your access) that leads to the kinds of situations you read about later in the news.
So I wouldn't worry so much about your DD being excluded. I'd focus the worries on how well she can handle the situations that can arise from participating.
I can say that when I was in the military, nearly all my friends were 3-5 years older, and it was a bummer when they went downtown to the clubs and I had to stay behind, but overall, it wasn't a big deal.
