I think the appropriate response would be a restriction of rights. Kids don't have a lot of rights, so anyone who can't mentally develop should not receive the rights and benefits of adulthood. But this is impossible without overcoming easier obstacles, such as invalids counting as adults and citizens.

Yes, you'll still get a bunch of them accepting the tradeoff. Other measures will be needed.

We don't even test for iq to allow voting. We should be testing for -something- to earn that right. Maybe this slipper slope has some consolation prizes on the way.