It seems like there's a turning point in every kids life when they see their first R rated film. For me, the rating means they didn't cut back, or try to soften something to make it more family friendly. I think everyone has that point, where they see a movie that is nothing like they've been allowed to see before, whether it has graphic violence, hard core sex, or explicit suggestions to go fuck yourself.
My first R movie was Office Space
It was the greatest movie I'd seen up to that point in my life. It was hilarious and used the English language to it's fullest.
Is this an 18 certificate movie? If so, then the first I saw at the cinema was Stigmata. In general, I think it might've been Evil Dead. Can't really remember, I was only about 5. I just remember, about that time, my much older brother babysitting me and my younger brother, and he put the film on. Nice brother I have. It scared the crap out of me at the time.