If I am 16 years of age, can I attend a movie rated NC-17 with my mom?
This is the big difference between R and NC-17 ratings. R rated films can be seen by people under 17 if accompanied by a parent, but NC-17 cannot get in under any circumstances. Another big difference between the two is that NC-17 films may not advertise in newspapers or have commercials on TV or Radio, so many filmmakers shy away from this rating so they are not limited in this way. The rating didn’t even exist until 1995. Before that, films with this much "inappropriate" content were rated X. However, X had become associated with pornography and any legitimate film rated X would be viewed as such. This really hurt some very good movies that just happened to have a lot of adult content, so the NC-17 rating was created.
If your interested, there is a very interesting documentary of the bias and unfairness of this system and the NPAA (who rates movies) called ‘This Film is Not Yet Rated’. Until the system is changed, you can still watch these films in your home. I would recommend it, since any filmmaker brave enough to leave their film unedited usually has something interesting to say.