In most states, the minimum age to work in a bar is 18 years old, but there are some exceptions. For example, if the bar serves food, you might be able to work there as a server or host/hostess at 18, but if it's a bar that only serves drinks, you might need to be 21 years old to work behind the counter.
It's kind of like trying to get into a theme park - there are different rules for different rides, and you need to make sure you meet the requirements before you can get on. In this case, the "ride" is working at a bar, and the requirements are set by the state and federal laws.
