No, I don't think any of that follows from that. The header can be set in the initial communication attempt. It is even more specific than any other mechanism to verify your age, because this header can be set appropriately for any web resource.

The problem of hosts in countries that don't give a shit is true for every solution aside from the great all blocking firewall no developed nation would want to have. So no to "ISP null routing" from me. ISPs provide infrastructure. They are not school teachers.

Such a solution implemented today would be tunnelled yesterday and everyone should support evasion attempts.