> but as I understand it, the requirement is that the system administrator assigns ages to the users on their system. That seems pretty reasonable to me, and maybe even like a good idea in some scenarios

Does it require exact age, or just a flag >=18 vs <18? It seems like this could be trivially met by something like a file /etc/userages, where if a login is missing from that file, it is assumed they are >=18 - and a missing file is equivalent to an empty file