I think the first R-rated movie that I went to see in the theater without parents was Terminator 2. I was about 14 or so, and nobody checked ID. I really felt like I was getting away with something.
This reminds me of a Simpsons episode (I can't remember which one) where a bunch of Bart's friends are in the back of a car driving by, and they say they're going to go sneak into an R-rated movie called "Barton Fink." They keep chanting "Bar-ton-Fink! Bar-ton-Fink!" as though this is going to be the biggest thing they've ever done.