Minimum drink age aboard a commercial flight in Canada

Interestingly it seems to be down to the airline, here's a Daily Mail article about and a Huffington Post piece.

From the first link:

Is there a drinking age restriction?
Despite restrictions imposed in bars and clubs, there is actually not a limit enforced during flights.
Individual airlines have to decide which rules they abide by, although it is usually the laws of the country where the airline is registered.
Richard Taylor, of the CAA Corporate Communications team, said: 'Cabin crew have the right to refuse to serve alcohol to any passenger. 'If they suspect a passenger is under-age then clearly they should not serve them.'

And from the second:

Many airlines explicitly note the minimum age they will serve alcohol to on their flights, which is almost always the same as the drinking age of the country the airline is registered in. For example, 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. It's not quite Tijuana.

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But there are exceptions, depending on where the plane is leaving from. Some flyers from a bodybuilding forum (hey, everyone loves to travel) report that airlines will follow the origin country's rule. So even if you're on Aeromexico going to Puerto Vallarta from Los Angeles, you might not be able to drink booze on the outbound but would be able to on the return trip. At least you get to somewhere, right?

So, you'd need to check with the airlines involved, a search for the airline name and 'drinking age' should get you the right page. For example, if I Google "air canada drinking age" the answer is right on the results page in one of Googles info boxes (or whatever they're called).

18 years Cabin crew must not sell or give alcohol to anyone under 18 years of age. However, on international routes, cabin crew may serve alcohol to passengers under 18 if they are accompanied by a parent or guardian who have provided consent. Any alcohol served on an Air Canada flight must be consumed on board the aircraft.

Here's the Air Canada page that that's from.

I can't find a handy list of drinking ages per airline so you'll have to search each one. Also note that those articles basically say that the cabin crew don't really ID people, so I'd suggest if you're over 18 and want a drink just ask for one when they ask what you want to drink. Just be prepared to be possibly ID'd and possibly refused (which is always the case). Also, be polite, if you do get served don't drink too much and if you don't then don't make a scene about it.