Things change all the time, but this is how it was last year. There was no alcohol at all until parents' weekend. That weekend, pretty much everyone who was legal got smashed. The rules say you can't have alcohol in the prep school area. The on base bowling alley serves beer a couple blocks away. Walking distance. That is allowed. You can drink off base, but remember, you need to get back to base, so have a DD. A lot of the clubs in the Springs area are 21+, but in Denver, there is Vinyl and Beta that are both 18 to get in, 21 to drink. You can have an underage DD at those places. Although you can't have your own car, a rental is allowed! =D
EDIT: Also, no alcohol when you have classes the next day, so only Friday and Saturday nights, unless there is a holiday or something.