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.

My (rather limited) understanding is that Arch can be installed both without the archinstall script and without a user. Also, the rest of your comment covers how stupid it is to require a value anyway since people can put whatever they want.

Outside of that, it's all open source. It's possible to fork and remove the field entirely from an install script, distro, or even systemd itself.

Nobody can enforce this in the open source world. This is honestly the strongest argument for an open source exemption in these laws. It cannot be enforced on open source OSs.