My first R movie was "The Jerk". I was staying at my grandparents house when I was about 10, and they let me and my younger brother watch it. I think it's rated PG 13 now as well.
I honestly don't recall the first R rated movie I saw. I do know that I'd seen Death Wish, Up In Smoke, The French Connection, One Flew Over The Cuckoo's Nest, and a ton of other R rated movies by the time I was ten. The rating wasn't ever a big deal to me until I saw 48 Hours. That definitely stood out from all the 70's R flicks I had seen...
I was about 5 and my parents and had goen tothe Beach (Jones Beach) and then the sky started pouring rain. We drove around, had dinner at Fiendly's and then hit a movie. That movie.. Blazing Saddles whci hhad been re-released t otheaters. The Year... 1977. My age: 5.