Here is the situation:
A person age 19 flies to the US from the UK. In the UK the legal drinking age is 18 but in the US it is 21. So are you allowed to drink on the flight?
Drinking age on airplanes in the U.S., at least for those in flight, is not set by regulation or law. The drinking age on planes in the U.S. is set by the carriers.
Each airline has their own set of rules for international flights which can be affected by the national laws of the countries over which they fly.
Some airlines set the age based on the country of registration, some based on country of departure and some by country of destination. You will need to check with your particular airline
For some airlines, the drinking policy changes depending on where the aircraft is located. As an example, I flew Air France out of Saudi Arabia and there was a celebration when leaving Saudi airspace because they could, or would, then serve alcohol.