Hm... Well, a lot of the movies I watch are R-rated. There are way too many splendiferous movies that are, unfortunately, R-rated. I'm merely 14 years old, but R-rated movies include most the good stuff that are coming out nowadays. Many of my top preferred movies are 'rated R for restricted'. (Battle Royale, Pi, Donnie Darko, The Machinist... To name a few...)
I started watching R-rated movies when I started watching movies more so for the purpose of inspiration for writing. I'd say around 11 or so. Perhaps even earlier, not sure.