Well personally, I don't think there is not a specific age to watch an R rated movie because the first time I watched was when I was in 4th grade (now don't judge haha). I think the question mostly is how mature a person is to actually be comfortable with the topic without making irrational actions afterwards. I should be giving advice such as self preservation but I think you get it.
I think R is the USA's equivalent of an 18 or a 21 over here, so I'd say 18 or 21.
People under the age of 17 must be accompanied by a parent or supervised by an adult.