I've been carded when using drink chits when I've been stuck in coach (I was 21 at the time) - I think it was always on Comair, but once it might have been on Chautauqua. I've been carded a few times getting a drink in the Crown Room as well.
Never been carded in F though. Once or twice, a F/A has asked me if I was indeed legal.
My younger sister was in F on a flight out of AUA with us once (family vacation) and tried getting the mimosa before departure - F/A did deny her the drink.