Actually, most European countries do no have a legal drinking age in the same way the US have it. In Europe, the law is normally just forbids establishments to serve alcohol to minors (at a certain age, depending on the type of alcohol), but does not forbid consumption of alcohol by minors. For airlines, this doesn't matter, since they will be bound by these serving rules, but it makes a difference in the way young people can drink. In most European countries, the police has no right to interfere in situations where young people drink únder private circumstances e.g. at private parties. since in Europe, the law is aimed at businesses, young people drinking commit no offence. Note, that the rules and ages vary by country, there is no uniform European law on the subject, but my perception is, that this is the general principle in most of Europe. Northern Scandinavia is stricter than the rest of Europe, though.