You’d think that one could simply block sites that don’t have the age header set on child computers. This may block kids from hobbyist sites that don’t bother to set their headers as kid-friendly, but commercial sites would surely set their headers properly. Over time sending proper rating headers would become more normalized if they were in common use.
This still isn’t perfect, as it creates an incentive for legislators to criminalize improper age header settings and legislate what is considered kid-appropriate. But it’s still better than this age verification crap.
Yes, that's how parental filters already work. They use a combination of rta tags and external data to block pages. Even works with Google safe search, firewall devices, etc. The rta ecosystem is already built out and viable.
An age header is not the answer. Why should a site have to decide what content is appropriate for a 18 year old and what content is not? Who is qualified to make that decision for every 17 year old in the world? Do they know my 17 year old? Do they know the rules in our home? What if I'm OK with my kid seeing sex-education stuff, but some lawyer at Wikipedia just decides to tag sex ed articles as 18+? Now I have a shitty choice: Open up the floodgates of "18+" to my kid, do it temporarily while the kid browses the sex ed sites, or not let the kid browse them.
Letting a company or government decide what's appropriate for what exact specific age is fraught with problems.