Since anything seems to go these days in the world of tv, it would help parents to have a rating system by age so they can decide if it is a good idea for their children to see a particular show or not. Of course it would be up to them.
As a child my father was not around much, and left my sister and I alone at the house. Of course, being children and having no one watching us, we decided to watch TV all day. Admittedly, my sister and I often watched shows that were not appropriate for children. (Homicide, X-Files, South Park, etc) So, yes I believe that shows should be rated by age. There are things I should not have been exposed to as a child. And I would not show my children until they're of age of clear understanding. I am glad that now-a-days you can set the television to have parental controls.
I watched many R rated movies when I was young, but with guidance from my parents I knew right from wrong. To keep a child from a story just because they're young, and don't look comprehending doesn't mean they are. It is a disservice to keep knowledge, and wisdom from a child just because of age.