What is the relevant age you are talking about.

For almost all of Europe, 16 is the age at which you can be served in a bar or restaurant. That INCLUDES the UK with regards to restaurants only, and I guess a plane counts as a restaurant for this purpose. So it is not at surprising that KLM are relaxed at serving people who look 18, well above the minimum.

Poland and Portugal are exceptions however, where the legal drinking age is zero. The cold and miserable countries on the edge of Europe, Finland, Ireland, Sweden and Denmark, insist on 18, presumably to keep the kids sober on their bicycles on the way to school.

The USA is on a different planet.

What rules apply on an airplane?