B..
For ME, the right age is when I think my son would be OK. He is 3. Right now, he has had sleepovers with his cousins and his best friend (my best girlfriend's son.) I can't imagine myself sending my son anytime soon, if I'd never met the parent's before. If I knew them, I would not hesitate. I was allowed to have sleepovers starting at 5, but I could not sleepover somewhere, if the parent's were strangers. Right about 7-8 parents felt like I was educated and confident enough to keep myself really safe. They weren't concerned as much them with knowing the other family well. I think I will approach things with my son the same way.
I think it really depends on the child. At 7, I could handle myself. I was not an easy target, because I couldn't be manipulated, or separated from the group. My sister, they did not let her sleepover until she was 10. It was for her own good. She was easily influenced, and manipulated. She was not a very "strong" child, and my parents decided she couldn't go until she gained those skills. Point is, some kids are ready...some kids aren't. The parents know their children.
Oh, I'm 30...if that matters.