I think you'll find that in every part of the American Airlines system that enforces a drinking age, that age will be 21. International destination/origination will not matter-- when AA chooses to enforce a "drinking AAge," it will be 21.

All of this said, many flight attendants do not bother to check ID/enforce the drinking age. Order whatever beverage you'd like, the worst the flight attendant can do is politely refuse service.