Flew UA837 (NRT-TPE) today.
Asked for champagne after take off as no drinks were served prior to departure.
The flight attendant asked me if I was old enough then told me the drinking age is 21 as per US regulations (I am 20, legal in both Japan and Taiwan).
This is the first time I have been asked my age on any flight.

I kind of feel that part of it was her being lazy as there was no champagne bottle on her cart; she'd have to go back to the galley for it.
Anyway, can anyone confirm the drinking age on UA and share their experiences?