Age verification at the content end has always been a silly idea. Eyeball networks can, and do, implement such filtering already - for example, UK mobile networks block porn by default, but allow any adult account holder to connect to unfiltered service.

Providing a BGP feed of such provider network subnets to content providers would then allow them to happily serve content to those subnets without any further checks, safe in the knowledge that they will only be providing service to endpoints controlled by adults.

Details of how this can be done for other services including home broadband omitted - suffice it to say your router would have both adult and child-friendly SSIDs.

This seems both simple and obvious, and protects children without encumbering adults, risking privacy, or forcing a mass censorship regime on everyone.