I am not fron the US, but I actually thought it was already 21 due to drinking age law. It's 18 to drink where I live.
Kinda weird to let people strip at a place they cannot be legally customer.
A couple of things:
You can work in an establishment that serves alcohol if you're underage to drink, you just can't be the one serving it (though this may vary from state to state)
In some states, the level of nudity is limited by the liquor license. For example, in Illinois, you could not have fully nude dancers if you served alcohol (**that is based on the last time I went to a strip club in Illinois, which was nearly 20 years ago)