Of course, you know your son the best. Some kids can manage listening/watching PG-13/some R-rated things, some can't. It's for each parent/adult to judge whether the child can manage it.

René Dif for the win!

Well, he isn't technically retired. He did appear in last season's episode "Let Go, Let Gov".

Old enough? I introduced my 6-year old half siblings (they are 9 now) to South Park, The Simpsons, American Dad!, Wolfenstein and Duke Nukem. I even swear in front of them. They are still normal kids (well, they like the before mentioned series/games, but they still criticize me if I swear). :P