The climate for underage drinking varies widely from place-to-place and school-to-school, even from day-to-day. Generally there is indulgence of underage drinking, in particular by college-aged students, as long as it is kept on private premises or on campus (or at least in particular areas of campus) and isn't so obvious, intrusive, or harmful that the authorities feel the need to do something about it. But there are enough exceptions to this general rule that local practice needs to observed and understood.
Underage drinking in public (at bars, etc.) is more problematic. Serving underage customers can result in penalties and even loss of license to bars and clubs that do it, so enforcement is capricious and can even depend on how your server is feeling about you at the moment. Fake ID's are, indeed, fairly common but using them is also a risk, and you can't know how savvy the person you are presenting it to will be. Personally I would not recommend this practice.