I mean, how is the OS going to actually verify the age of the operator?
I see how this helps Facebook - if you lie to the OS, and the OS tells Facebook that you're over 18, then it's not Facebook's fault if they provide you an 18+ service.
I don't see how this helps anyone else.
That's the difference between a parental control and a pinky swear.
You are assuming the parent is the administrator of the computer.