Age verification itself isn't such a bad thing. I feel most people are more angry about having to verify their actual identity. Every ad provider knows your address and complete identity every time you log into anything though. I guess its the illusion of anonymity that's so popular.
Age verification is an excuse for identity checking.
Horseshit.
There's often a degree of uncertainty with the data advertisers have. This would heavily reduce that uncertainty and enable worse behavior on the part of advertisers.