I am very mature for my age and started being curious about the other sex's bodies around 9 or 10. It was completely innocent. Dont know if that helps. But I think its natural to be curious at that age.
I think a person's innocence can vary due to their upbringing and their exposure to sexually things.
I had seen porn when I was 6 :/ I stumbled across it when I was flicking through tv channels.
I was very curious about the human body from then on. Although I didn't see a man's body until I was like 13.
When I was about 5/6 there was a guy my age who used to touch me down there during maths. I've never been able to work out what to make of the whole thing to be honest. I think it's more common than people like to think but I just really do not know what to make of the whole situation.
I think young children have sexual pleasure and obviously instinct tells them to act on this sometimes. They don't yet understand the idea of boundries. It's a complicated issue :/
I remember playing doctor with a friend when i was maybe 7, but i think there were times earlier than that too. as long as its with someone about your age who hasn't been interferred with i don't think there is any age.