Yeah, I don't make the rules and since I'm well over the drinking age, doesn't impact me much. The planes/operating license are still US-based. I don't know enough about the details, but even if they could, they may just have a blanket rule of "21" to avoid the complexity of country and regional specific laws. Sadly, its easiest to take the lowest (or in this case highest) common denominator. This could be as much UA policy as it is anything else.