Well if you are caught drinking or intoxicated under the age of 21 you CAN get charged with underage drinking.
Say a 19 year old boards a Canadian flight in Toronto after legally having a beer or two in the lounge, then consumes a couple more enroute to, say, Boston, will they have committed an offense upon arrival? I don't know the answer to that question. What I do know is that the laws of the country where the carrier is registered apply while on board. 19 year old flight attendants serve alcohol, legally, on that same route, as well as many others.