And it won't unless something else tells it

It can store your location data (i.e City/Address), because this service is specifically a user database. The systemd init isn't storing your age anytime son.

It can already store location data and other random metadata

The systemd service in question is probably already managing your accounts (if you've got systemd, that is)

SystemD is only adding the possibility to store an age for the user, and the PR is being debated still