Don't get me wrong, I'm all for privacy. But between setting up the birthdate when creating my children's local account on their computers, and having to send a copy of their ID to every platform under the sun, I'd easily chose the former.
I'd even agree to a simple protocol (HTTP X-Over-18 / X-Over-21 headers?) to that.
Whether it should be government-mandated is one question, but it seems more like this “it’s the parent’s job, not the OS’s” has become a tagline that people just repeat rather than really thinking through it.
People usually repeat it for calls for checks on online platforms, not just within the scope of OS - but its primarily stated because people, quite fairly, don't want to see websites having to shut down, or them losing access entirely, or losing access without handing over private data because of parents inability to control their childs access to the internet.