How young is too young for a child to be...self-exploring?

I am only asking because I answered in another question that an 8 year old was too young to be masturbating and this may want to be mentioned to a doctor. Well, I got only negative responses. Am I crazy to think that is a little young for such behavior? I realize some self-exploration is normal in young children, but isn't masturbation something kids usually start around puberty?