According to Bleeping Computer, the company behind PornHub has blocked their services to states that have begun to require adult websites to verify the age of visitors.

I file "age verifying website visitors" under "good idea in theory, but probably really hard to implement in a way that both works and respects privacy of individuals". I had a good discussion with someone about a related topic just the other week. Their general position was that age verification as a service would be an incredibly beneficial and viable business model. My counter was that, although good in theory, I seriously doubted if it could be effectively implemented without trade-offs to either privacy or limiting reach of the program in a way that made it not financially viable.

Something I hadn't considered but the BC article points out is that this could decrease safety by encouraging visitors (even those who shouldn't be accessing them in the first place) to visit less-trustworthy websites to get their "entertainment".