Joseph:
3 points:
1) States and more commonly, municipalities, have laws against minors in possession and/or minor consumption.
2) Even if, in a hypothetical jurisdiction, there is no law against underage consumption per se, there will be a law against providing/serving alcohol to minors. Often times, adults who serve to minors are held liable for anything the minor does while drunk. If the minor gets in a car and kills someone, you’re looking at a LOT of trouble.
3) The religious exception is for sacramental wine used in religious services. Kiddush would be a good example. Purim would not likely fall under such an exception – there is no religious ritual or ceremony involved. Additionally, as there exist opinions that say one needs not necessarily drink on Purim, no court would likely extend the exception on the grounds of necessity for free exercise purposes.