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.
The problem with that is that sites/apps will retain the identifier, either to use the Digital ID for login (not just one-time age verification), because they want to retain as much information as possible for later usage or sale, or because a government told them they have to retain it so all their social media activity can be easily linked to them.