I know systemd is making moves to add “age verification” in the form of an environment variable or something

That's not true. Systemd isn't making moves to add age verification. They added a field to STORE a birth date, which is not enforcing anything. It's like a field in a database. It can be there, but you don't have to use it.

Systemd is not in the position to do anything. The age verification has to be implemented by the OS vendor, not by Systemd. That lie is spread by people with very little knowledge of how linux actually works.

Im just curious, but how would arch implement age verification if it wanted to

No idea, but there are lots of ways. You could put it into the archinstall script and just never finish the installation if there is no age set. You could also prevent a user from logging into an account that has no age set, this could be achived by modified core packages in the base package.

However, you will ALWAYS be able to circumvent it as there is no central server to manage it. Nobody is stopping you to put 01.01.1970 into the birthdate field. It's basically as pointless as cookie banners.

The issue about this age verification shit isn't the present, but the future when governments start extending the laws that, at this point, are already in place.