It's literally an optional birthDate field in a place where there's already realName, emailAddress, and location. If you're concerned about privacy, maybe don't expose your real name, email, and location.

And it's not even fucking installed everywhere:

$ userdbctl
Command 'userdbctl' not found, but can be installed with:
sudo apt install systemd-userdbd

Anybody who is calling this age verification is actively lying to you!

This Sam Bent guy should fucking get bent.