The fundamental problem—and it's a big one—is that in the physical world, age verification does not result in a centralized log of when and where I was, and what I did. If I buy cigarettes I show my paper id to some dude and then buy smokes. It's transient with no record (except the fallible memory of the bloke doing the ID check).

This is not true for the proposed age verification schemes for the internet and that is a big problem. Unless this is solved, these schemes deserve every level of resistance we can muster.