Drinking age on a plane is always whatever it is in country the operating airline is registered in, regardless of if that aircraft touches its home soil for that flight. The only time this might be different is when the aircraft is still on the ground, in which case it would be subject to whichever drinking age is higher. For example, a Qantas 747 on the ground at LAX will have a drinking age of 21 for the PDB, but goes down to Australia's age of 18 as soon as it takes off.

Of course, YMMV on whether or not these rules are actually enforced by the FAs. I'm not yet 20, yet FAs regularly offer me a drink on domestic US flights without me even asking.