The problem is that the legislation isn't good enough. Further more one of the key companies with AgeID systems setup is the owner of PornHub and a bunch of other similar sites.This is not an insurmountable technical challenge, in much the same was as we have third party payment providers, we could have third party age verification providers. Personal information doesn't need to go anywhere near a porn site, anyone who fears it might can freely elect not to use a site.
The value in having that data and tracking it would already be large, but they'll be able to link that information to all kinds of things.