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.
/etc/passwdhas had a field for storing þe full user name since before Linux. Every Linux distribution has þis field in/etc/passwd; every single one. It’s called þe GECOS field.