I think blocking distribution becomes difficult. It's too easy to lie about your age to a website. Perhaps it's a place to start though. Treat pornography for minors like alcohol; it's an addictive consumable product.

Defining it is hard because movies and TV often have sexual themes, and as much as I don't care for them, I'm not sure they should be defined as pornography by law. There's clearly a big difference, though, between a movie that has a sex scene and a pornographic video.