I agree with you. I would start it at about 12 or 13 years of age. I would also emphasize it more as a sport than "self-defense."

Competition, loosing and winning, are both parts of life that should be learned. I would say that how you loose is more important than how you win.

I feel that a 6 year old who knows how to choke out another kid might be too dangerous because at that age they are not able to discern when not to do it or worse, when to let go. It's common sense to me, like the same reason we don't let 6 year olds carry guns. No matter how much you teach them about it, they still can't be trusted alone with it.