Paradoxically this is one of those features/requirements that i feel should be on the end-user-device with zero knowledge proof. It would make sense to have the enduser verification ondevice with a simple reply to any online property : Passed age verification/or not. Otherwise the centralization and eventual leak of this data is a can of worms in waiting.
Big tech did not want to cooperate to do this for some weird reason so now we get a much more complicated solution.
Yes I know that if your kid uses a live USB stick he could watch porn on his laptop but IMO is much easier for such a smart kid to find a website that does not respect the browser headers and torrent adult content.