This is sort of off-topic, but there's a bar where I went to college where every bartender except for one (and at that, only right before he got fired) would allow anyone with a college ID to drink, except in exceptional cases. These exceptional cases have been, in my experience, limited to non-regulars who showed up drunk, crazies, or people who otherwise seemed untrustworthy. Generally, it has little to do with age when someone gets denied.
It's sort of an unwritten code that, as long as you're not a high risk for causing the cops to show up, you can drink. The regulars go pretty far out of their way to neutralize bar drama before it gets out of hand - there were a few times that bar fights should have broken out, but didn't 'cause we'd rather be drinking than talking to cops. Last weekend, there was some '08 rand that got up in the bartender's face while he was trying to break up an argument, and something like half of the people there instinctively jumped him and threw him to the curb.
