It'd be the sensible non-surveillance option. All that would be required is that the OS or SIM sold is a tiny bit different with a flag that the browsers/apps respect and that each website that accepts underage users sends a header that browsers can check and allow/disallow access. Compliance with child friendly legal requirements would still be with the company owning the website.

Of course it's not as attractive to the governments as putting in place a full scale surveillance aparatus that puts KGB to shame.