Something like that parody of printed manual check copy protection on the original Larry game might actually work? Years later I had a huge "oh, so that's the answer!" moment when I finally discovered the meaning of the words agent orange.

Jokes aside, what would be the failure mode if some metadata about audience minimum age was given legal blessing? Regular browsers would simply ignore it, browsers with enabled parental controls would reject the page or site. Vendors of parental controls could focus their efforts on suppressing those sites that lie and the numerous proxy services that would inevitably pop up (but that's a problem they already have I assume). What am I missing?