It's been illegal to sell porn to minors since approximately forever. If that is constitutional (not saying it is, but I'd be surprised if it wasn't since it's such an established practice), then I don't see how requiring age verification on porn sites wouldn't be. Requiring health warnings might be another matter, though. Not sure about that.

Does the “sell” part matter - like is it simply that the sale to minors can be regulated? If it is free isn’t it just transmission of information?

No, it's still illegal in most places to knowingly distribute pornography to minors - the law in my state starts;

> A person who sells, gives away or in any way furnishes to a person under the age of 18 years a book, pamphlet, or other printed paper or other thing, containing obscene language, or obscene prints, pictures, figures or descriptions tending to corrupt the morals of youth

I wonder how it relates to the health warnings on tobacco products?