Experimentation with moves from Dad's Jiu Jitsu book in school when I was 10-12. The headmaster warned me not to bring it to school again. I didn't attack anyone, but a classmate asked me to demonstrate one of the moves on him....Anyway it was 20-3 years ago and while I haven't met him since left Primary School I wouldn't say he holds it against me. He backed me up with the teachers that we were both in on it, and his older brother sortof accepted that but was clearly quite cross with me.
I still feel very dumb about it.
Am fairly sure I was below 12 when it happened because I was 12 in my final year and his older brother was in a year ahead of me at the time. Children are very impressionable and curious and should be shielded from that kind of influence.