How old do you have to be to watch Unrated movies?

To my knowledge, there is no law prohibiting the sale or rental of R, NC-17, or Unrated films to people under 17 (in the United States, at least). Rather, each video store sets their own policy. They may choose to rent/sell R, NC-17, or Unrated films to minors, or they may choose not to.

So, by law, I don't believe you are required to be any specific age to rent or buy an Unrated movie. But some stores have their own policies against renting/selling R, NC-17, or Unrated movies to minors.