You can say "well don't put it in there" but what happens when big monopolistic corporations start requiring it to use some service of theirs that you don't have an alternative for? Now I have to maintain a separate PC for that shit? Fuck that.

If you choose to use a service that requires age verification then that service will store your age verification information on your computer somewhere. If it is stored in systemd, malicious programs will be able to access it. If it is not stored in systemd, malicious programs will still be able to access it.

If you choose to not use a service that requires age verification, then you will not store any age verification information on your computer to be stolen by malicious software. Even if systemd has a birthDate field you will not store any age verification information.

The difference in these two scenarios is your choice to use age verification or not. The location where the data is stored doesn't change the scenario.