> But the only way to do this is to require ID checks

Not necessarily, consider the counterexample of devices with parental-controls which--when locked--will always send a "this person is a minor" header. (Or "this person hits the following jurisdictional age-categories", or some blend of enough detail to be internationally useful and little-enough to be reasonably private and not-insane to obey.)

That would mostly puts control into the hands of parents, at the expense of sites needing some kind of code-library that can spit out a "block or not" result.