This exact method was implemented back around the turn of the century by RSAC/ICRA. I think only MSIE ever looked at those tags. But it seems like they met the stated goal of today's age-verification proposals.

That's why I have a hard time crediting the theory that today's proposals are just harmlessly clueless and well intentioned (as dynm suggests). There are many possible ways to make a child-safe internet and it's been a concern for a long time. But, just in the last year there are simultaneous pushes in many regions to enact one specific technique which just happens to pipe a ton of money to a few shady companies, eliminate general purpose computing, be tailor made for social control and political oppression, and on top of that, it isn't even any better at keeping porn away from kids! I think Hanlon's razor has to give way to Occam's here; malice is the simpler explanation.