We had someone with our group in this exact situation, and yes, they take note of age, it's in the computer - I think he had a choice of hanging out with the teens, or being in the "adult only" areas, but not both? (adult only = 18+) As in they didn't want that crossover - either be with the teens, or take part in some of the "adult" nightclubs.
Don't know how they'd have checked actually...unless it was a reservation at Palo's or something, where the name is in the ressies and therefore it's a quick check of the computer...