What age could you be an actor/actress in a rated r movie in 1980?
Any age. Though child nudity is not common but there were quite a few movies that showed underage nudity. A movie called Once upon a time in America(1984) showed a 10 year old girl naked. There was few other movies I can think of that showed younger teens naked.
Brooke Shields was famously in "Pretty Baby" and Blue Lagoon..... and Jodie Foster played a prostitute at 13 in Taxi Driver. Can only happen with parental consent. ...who pimp their kids out like that.
There's never been any limit on that. But even the actors wouldn't be allowed to see the entire movie if they were under age.
There were lots child actors in R rated movies. They obviously weren't in any nude scenes, but they were in the movies.