For those interested on the question of age verification and GNU/Linux: be aware that Systemd v261-rc1 was recently released. It now implements an optional birth date field in the JSON user database (see second item under “Other changes”).
The implementation of this field was prompted by age-verification or -attestation laws.
This is a take I think makes the decision make more sense to me (if this is the reasoning), letting the user opt into compliance rather than forcing their hand.