If the bar is restricted to guests only, then it is pretty easy to control the drinking age.
When people check in, you take their passport details as part of the check in process (and check their date of birth) so you know which people are underage and who can drink. You can then update the bar staff with details of who is underage so they don't need to keep checking everyone's ID.
Depending on the market your hostel is going after this can be a problem in the US. It wasn't till my third trip around the world that I was old enough to drink in the States; however the majority of guests will probably be old enough to drink in your hostel. It would be interesting to see what percentage of guests are aged under 21.
I think there are a lot of opportunities to run a hostel in the US, but I would be reluctant to run one with a bar in the States.
i think we're on the same level there. as much as i want to include a bar, i dont want to exclude too many guests. but that said, how many under 21s backpack around the states, given that they cant drink? i know that may seem like a silly reason not too, but visiting bars and meeting the locals is a big part of travelling and it certainly would've frustrated the hell of out me had i gone to the US when i was younger. maybe time for a bit of research into the age of backpackers in the states...