In the case of the young girl that I knew, the boy was a 14 year old brother of her friend. It started as a childish exploration/exploitation, and progressed for three or four years without being discovered. It was mutual, but the girl was too young when it started to even know what was she was doing. The boy was about four years older.
A few years back there was a underage girl at a party with some friends & she said a group of boys touched her. She went home told her parents & a few of the boys were arrested. One of the boys hung himself before it came out that she was out later then her curfew & made the story up so she wouldn't get into trouble.
Those are the stories that really get to me.