The legal drinking age in the United States is 21, and this applies to bars as well. If the bar is part of a restaurant, those under 21 can still eat there, but they won't be allowed to sit at the bar. If the establishment is strictly a bar, you will likely be asked for your ID at the door. It all depends on the specific bar. Some bars require everyone who enters to be of legal age, so they don't have to ask for identification every time someone orders a drink.
Other bars allow minors in all the time, while some only allow them at certain times (usually when there are fewer people). Even if minors are usually allowed, some places don't let them in during big events like the Super Bowl or major soccer games. Minors can sit in the bar, but waiters advise against it since alcohol-free customers are unprofitable. It's important to note that during important sporting events like the Super Bowl, it will be much more difficult (though not impossible) to find a bar that lets minors in.
