My college team was able to pull off a beer mile at our track with about 15 participants and 20+ spectators. I don't think having 30-40 people running would be any more of a problem other than a crowded track and the fact that you would need at least 120 beers ready at the start. If you can do this at night, on a track that is not on a main highway, then that's the way to go. You're gonna want it on a track though, man. That's where the magic is. If you're worrying about getting minors drunk and getting in trouble for that, then you aren't cut out to run a beer mile. I'd say the majority of beer miles are run by college teams, and the majority of college teams are under age 21.
Beer mile in college = team bonding and hilarious stories you will tell for a long time
Beer mile after college = borders on pitiful, unless you are in range of a record or something, or are in some kind alumni beer mile event.