For what I understand, OpenBSD could just patch useradd so that the age category is mentionned in the comment field of /etc/passwd or a random text file in /etc.

Haiku could just run an automatic dialog asking you if you are minor, in Illinois or California and write a text file with the corresponding age category of said person.

These bills do not mandate that the user cannot modify that information AFAIK.

I can't imagine OpenBSD would be bothered by laws specific to a very small selection of US states.

By adding a simple birthdate field to your account info and a system API of some sort for retrieving the account owner's age range, same as everyone else.