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.
Once again for the stupid…IT IS OPTIONAL. YOU DO NOT HAVE TO PUT ANYTHING IN THERE NOR DOES ANYTHING YOU PUT IN ANY OF THOSE FIELDS HAVE TO BE THE TRUTH.