I am a life-long teetotaller from a country that allows alcoholic drinks if served with a meal to be bought in restaurants by 16 year olds, allows 18 year olds to buy and drink anything they like in bars and pubs etc and in the home, allows parents to give alcoholic drink to 5 (not 15, 5) year olds. I have seen many lives ruined by alcohol, one of the reasons that I steer well clear of the stuff. However I do think American laws are strange to say the least and inconsistent with other age limits.
But the law is the law, we might think a law is strange, even silly, but if it is not unjust or sinful, we are required to obey it, even if we are actively campaigning to change it. So it is sinful if you are in a part of the world in which the local law has a drinking age of 21 to encourage or assist under 21’s to drink, or to drink yourself if you are under 21. Using or facilitating fake ID adds an extra sin.
Not saying that this is your attitude, but an attitude of ‘this is only a venial sin, therefore I will do it’ is itself wrong, and damaging to yourself.