A Proposal to make the 'age verification' forces happy -- with no OS changes at all.

It’s pretty simple, really just a proposal to standardize on an admin policy. No new APIs, works with any legacy system (systemd or not), not even Linux-centric, pure POSIX/UNIX. Here goes:

Define standard groups which *may* be present. Make each user a member of three groups: YOBnnnn, MOBmm, DOBdd, where:

YOBnnnn = Year of Birth, eg. YOB1981
MODmm = Month of Birth, eg. MOB01
DOBdd = Day of Birth, eg. DOB03

So to define an age for user 'jsmith' born Jan 3, 1981, just define three groups YOB1981, MOB01, DOB03. add user 'jsmith' to these 3 groups.

Only a system admin can add users to groups, so this ensures it's admin-enforced and users can't change their own age verification.
The groups can be defined only as required, so a system with only a few users will only have a few of these YOB/MOB/DOB groups defined and it's easy to add or remove them at any time.
Querying a user's age is as simple as calling standard POSIX APIs to see which groups a user is a member of, and extracting the numerical suffix of any groups of pattern {YOB, MOB, DOB}.

If a system doesn’t want to do age verification, just don’t define or assign these groups to anyone; it’s up to apps to assume the age is either satisfied or not when groups are absent for a user.

If privacy is a concern, assign only YOB (or YOB and MOB), not DOB group, so exact age is not defined (with the loss of some precision, eg., someone 17 years of age isn’t considered 18 until the start of the following month after their real birthday if a DOBdd group isn’t assigned to their user).

If age verification were required to be done by ‘age classes’ rather than YOB/MOB/DOB, define a group corresponding to one’s legal region requirements (I’ve heard California wants: ’13 and under’, ‘14-17’ and ‘18+’ or something like that). So just define groups ‘AC0’, ‘AC1’, ‘AC2’, ‘AC3’ for each age group, and assign users to one of those groups as appropriate. Again, apps just need to use standard APIs to check what groups a user is a member of in order to determine their age group.