I did a quick check of the FARs. They appear to only pertain to the crew. Is there some regulation stating that the alcohol laws of wherever the airline is headquartered apply inflight?
As far as I have been told before by airline folks, the drinking age on any flight is dependent on the carrier's Flag. DL is a U.S. carrier, hence the drinking age on any of their flights is 21. This includes international.
I had one FA on a regional flight ask me, are you really 21?, but I've never been asked for an ID. I usually make them guess my age, since I'm 32, but many think I look 18. 99% of the time I'm ordering alcohol in First and they never ask.