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.
That makes sense. Papa having control over what junior sees is probably not very functionally different from Big Brother having control over what an individual sees, lol