All of my kids devices are identified, at device level, as children's devices. They could've trivially exposed this as metadata to allow sites to enforce "no under 18" use. However, I'd disagree that my bigger concern for my kids isn't that they'd see a boob or a penis, but that they'd see an influencer who'd try to radicalize them to some extremist cause, and that's usually not considered 18+ content.

And either way, none of that requires de-anonymizing literally everyone on the internet. I'd be more than happy to see governments provide cryptographically secure digital ID and so that sites can self-select to start requiring this digital ID to make moderation easier.