I am not a baki in the halachos of dina d’malchusa dina, but I am under the impression that if the government is not makpid, it’s not a problem.
If it were, we would all be in trouble, because there are a lot of strange laws on the books that no one follows.
In Lakewood, the law enforcement is well aware that boys under 21 drink on Purim and they do not tell them not to – they just tell them how to drink responsibly. I am sure that the same thing is true in any community with a large Jewish community.
I have never heard of anyone getting arrested on Purim for drinking or giving drinks to minors.
Note: I am not giving an opinion about whether or not one should drink/get drunk as per halacha and/or safety issues, since I don’t know enough to have an opinon in those areas. I am only commenting on the dina d’malchusa dina which I am pretty sure is not an issue.
I don’t think I have ever heard of any Posek who said that it’s assur to drink on Purim for that reason. The issues are: 1. What is the halacha of drinking on Purim? & 2. Is it dangerous?