THIS is the solution! Promote it!...
Actually a simple user group "adult" that you add users to who are adults or "underage."
It's not OS enforced; it's STANDARD for software to poll to find out permissions. Engineering wise, shouldn't you use the OS existing permissions system to do this?!!
The law simply applies to software using a simple standard mechanism to determine it. If you want intelligence and security or whatever, those are 3rd party software that handle the user group.
The admin has a JOB to do. That JOB is to set access permissions. If the child becomes an adult, it is the admin's JOB (as a parent?) to set permissions. It should not be automatic or set by age or whatever. That is open ended and is entirely the admin's JOB! If an admin can't handle this simple task, they are too stupid to have children. Child protection services should take away their children; seriously!
Use case for those lacking creativity, a school. A school bans things from children. A child turns 18 but who is still in school and their peers are not yet 18 and the school don't just immediately allow porn or beer in the school at their birthday. An automated date rule would not work realistically in the REAL WORLD. Increased complexity (date+rule) resulting in even more complexity and special case exceptions. Poor design ends up in hacks and data corruption.