In elementary school no less:
In second and third grade, I was “mentored,” as it were, by an older girl in my neighborhood with whom I often played. She was about five years older than me, putting her in middle school. She had some very sadistic sexually-themed fantasies that she liked to have me help her act out. I can still recall these in some detail. Themes included luring/kidnapping, physical and sexual torture, bondage, lots of threats, and finally, cutting, amputation, or outright murder, often with cannibalization. I sometimes played the victim, sometimes the co-conspirator in these roleplays. I always identified with the dominator, though. It was my only experience at bottoming, in a way!