So, what’s the legal drinking age on planes?
Basically, the FAA has no rules covering the serving of alcohol, so it’s up to the individual airlines. Most adhere to the laws of the country in which the airline is registered, while others to the laws of the country from which the plane is departing. Still, others observe the destination country's rules. Yea, you can see why this is a confusing mess. As long as the plane is on the ground, though, local laws usually apply.
What do you mean by "where the airline is registered"?
Pretty much what it sounds like -- airlines are based in certain countries and thus, registered there. As it applies to booze, say you're flying from France to the United States on Lufthansa, a German airline that's registered in, you guessed it, Germany. As soon as the doors close, you’re technically “in Germany” and thus able to drink beer and wine at age 16. Those crazy Germans.
Like with gambling in "international waters," is there such a thing as "international airspace"?
Sort of. Airspace is controlled by the country it's above, but when it comes to booze, all space is pretty much "international." For example, a western carrier flying into Kuwait -- a non-alcohol country -- can serve drinks right up until touchdown. On its way out, only once airborne.