flatbush27:
As long as the provider of drinks to those under 21 is ready to go to jail and/or be sued due to their actions, they can serve someone under 21. 21 is random, as why not 20?
The problem with drinking at night is that you are not prepared (drunk, hung over, no longer in the mood, etc.) to be Mikayem the Mitzvos Hayom. Since there is no Mitzva to drink at night, someone who cares about Mitzvos should not put themselves in a matzev where they might miss out on Choivas Purim.
Josh31: Rav Zeira refused to go back, so even then its not a good idea.