[overboard2]

You have to pay an ISP which knows where you live to get internet. I think any potential age attestation stuff should happen at that level, with websites being able to query ISPs on whether a given ip address belongs to a child or not, and ISPs allowing customers to opt in to child-labeled ip addresses.

[dTal]

This already exists more or less exactly as you describe. Family mobile phone plans offer the ability to designate a number as having parental restrictions, and the phone company will intercept DNS requests and substitute a version of Google with SafeSearch enabled and locked, provided by Google for this purpose along with a friendly message "your network operator has locked your safesearch settings etc etc". Don't let anyone tell you this isn't mainstream.

[zamadatix]

A wired connection to a house in the US usually only supports a single IP to be shared by everyone in the house. Kids also access the internet from more than their home (e.g. libraries), so this also runs into overlap elsewhere too.

[dTal]

Mobile phones rather famously tend not to be plugged into landlines.

[nancyminusone]

not with ipv6

[zamadatix]

That's the unusual part (for wired connections in the US at least) and it's story isn't much better, it just didn't make much sense to detail its "why"s when the usual part (IPv4) already doesn't work for much simpler reasons.