The legal drinking age in the U.S. is 21. The age is set by the states, but the federal government enacted "incentives," like highway funds, to get all states on the same page. In this video, bartenders, some more experienced than others, show how they can tell if someone is over 21 and legal to serve.
Turns out they aren't as adept as they thought. That's why you get asked for your ID, even if you are 35. To be honest, people between puberty and middle age can be any age if you don't know for sure. -via reddit