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.
It’s a field in the users database on your own device. You can change it yourself. If something stupid like that would happen you could bypass it entirely by just setting the field to something else.
It is not early implementations I fear.