> Still weird though, since according to the MPAA, 17 is the cut off age for all movies, not 18...

Well, sort of. Last I checked, the MPAA guidelines were a little screwed up. To see an R rated movie, you supposedly have to be accompanied by a parent or guardian if you're under 18. But NC-17 movies can be seen by anyone 17 or over, without accompaniment.

It's all consistent unless you're 17 years old, when you can see NC-17 rated movies, but not R rated movies, without accompaniment.