Do you think that would prevent or discourage age verification software from existing? It's not as if a systemd user field is the only place a user's birthday could be stored.

Realistically, age verification software that is seriously attempting age verification isn't even going to touch the systemd field, because why would it? The field could only be trusted if it is managed by an age verification service anyway, in which case the service could just as easily store the data outside of systemd.