Hi

I'm pretty sure your assumptions are correct. No drinking allowed under 21 in USA based airline lounges, but ok to drink in lounges in countries where the drinking age is 18+.

However - on the airline issue I guess it may depend on the airline/crew.

I know for example that Cathay Pacific has a rule that alcohol consumption is NOT allowed for under 21s on flights to the USA (and I guess maybe the reverse too). I'm trying to find the link but not having any success. If I come across it again I'll post it.

Certainly airlines flying to another country can be bound by the laws of that country. I guess it would be a condition of their operating licence. For example all airlines flying to the USA had to have the 'no congregating' rule (some enforced it better than others!).