It's also unclear what they'd even be useful for to begin with.

You need some kind of proof system if you need a central authority to certify something, but why is that required? The parents know the age of their kids. They don't need the government to certify that to them. And then the parents can get the kids a device that allows them to set age restrictions.

Whether those restrictions are imposed by the device on content it displays (which is the correct way to do it) or by the device telling the service the approximate age of the user (which needlessly leaks information), you don't actually need a central authority to certify anything to begin with because either way it's just a configuration setting in the child's device.