Underage drinking is illegal and yet if halacha requires drinking, we would be required to follow the halacha against secular law.
Now according to many poskim, one is required to get drunk on Purim. But according to (at least) 3 heavy hitters- the Rama, Pri Megadim, and the Mishnah Berurah- you can be yotzei (and the M”B even writes that it is ideal to be yotzei) by drinking more than usual and going to sleep but there is no need to actually getting drunk.
Now, from a secular law perspective, for someone under 21, both of these options are technically illegal (I think) because they both involve underage drinking. Once you are breaking the law by following the halacha, from a legal perspective, is there a reason to choose the option that requires less drinking?