I have been to a few places where they card everyone and I mean everyone who orders a drink. The reason is that some people get offended by being asked when they saw someone else not asked. So the rules apply to everyone across the board and then the judgement call is not made by the wait staff. Many bars around town will ask everyone because if they allow an underage person in the bar they can lose their license and be fined heavily. Then if the underage person drinks, drives and hurts someone the bar can be held liable.
Lol. That made my day!