I'm 20, look like I'm about 12 lol, and have never had any problems. Just handed them the AMEX along with boarding pass and ID, and they type a few numbers in and that's that. I really doubt you'll have any problems. Never tried to get alcohol from the bar, I imagine if you sit down and start ordering they might ask for your ID and if they see you're not 21 you could have a problem, but as for entry to the club I've never had a problem.