score:3
For most practical purposes, an international flight will, when in flight, be considered in the jurisdiction of the state where the aircraft is registered. That is set by the Tokyo Convention you are referring to in Article 3, paragraph 1:
The State of registration of the aircraft is competent to exercise jurisdiction over offences and acts commited on board.
This not only applies to the passengers, but also to the crew. Therefore, the crew must also follow the laws of the state of registration, also laws on serving alcohol or making alcohol available to minors. There may of course be exceptions from this general rule, as a simple example, national laws may explicitely define different drinking ages for flights and on-land serving and airlines may of course also operate with higher drinking ages than required by law if they wish to do so.
Upvote:1
There are no specific rules as to what the minimum drinking age is (no official law), individual airlines can choose their own rule, usually the age that applies to the country where the airline is registered. So to find the drinking age you will need to check with that specific airline.
For example, from this HuffPost article:
If you are flying American Airlines, the legal drinking on board a plane age is 21. This rule still applies even if the destination is a country where the legal drinking age is lower. If you're headed to Mexico where the legal drinking age is 18, the legal drinking age on the plane will still be 21.
Also see this BBC article.
Some airlines hold permits to reduce the drinking age on their flights. For example, onboard Emirates Airlines, the legal drinking age is 18 as they hold a license issued by the Government based in Abu Dhabi where the drinking age is 18. In Dubai, the legal drinking age is 21.