If the proof of adulthood scheme is truly anonymous, one adult with some technical chops who thinks "kids should be allowed to watch porn if they want" would be able to, say, run an adult-o-matic-9000 TOR hidden service that anyone can use to pinky promise that they are an adult without fear of repercussions. If such a service comes with a meaningful risk of being identified and punished, it is by definition not anonymous.
I suppose I'm just not convinced giving up some basic liberties for a law that converts into sternly worded advice if just one adult chooses to break it is a great idea.
I also don't think you'll find many ISPs terribly keen to fight for the neutral treatment of TOR connections when the reason for this fight is explicitly to serve porn to minors.
For example, in the UK it’s only illegal to give alcohol to a child younger than 5 years old.
France has no limitations, giving a toddler wine is not explicitly illegal. Getting a child drunk would be.