Mine started pretty severely. I was maybe 11 or 12, just started high school, realising that boys had other things on their mind than playing tag in the playground (which i didn't like at all)...
I remember watching a horror movie with my parents. Later that day I began to think about being trapped by the bad guy... him forcing his hand places it shouldn't have gone.
I don't really remember the extent of the groping (not down there), but for my earliest fantasy it was pretty intense.
My parents laugh about that time, they still think I had nightmares about the movie for weeks and that's why I was acting weird. Looking back I probably just felt guilty for thinking about it!