I had probably seen other R-rated movies before it, but I still distinctly remember watching Alien 3 on television in 1997, when I was 6 or 7, and being completely confused (and slightly exhilarated) by the violence on-screen. I'm still an Alien fan to this day.
First R-rated movie I ever saw in the theatre was the terrible Tim Robbins/Martin Lawrence comedy Nothing to Lose, also released in '97. My brother (who was three years younger than I), my mother, myself and my grandmother all sat in the cinema shocked at what we were watching, yet we still stayed through the whole film. LOL!