I think another reason Krav should only be taught at a mature level is the mentality of it. It's hinged on survival no matter what, so you only learn techniques that will help you escape a confrontation or quickly beat down an attacker. It is simple, ugly and requires you to make yourself believe it is life or death. I don't like the idea of a 14 year old picking it up and using it to get even in the cafeteria at high school. I know personally after getting into krav, I want to fight people less than ever because what I know could really damage them. KM is alot like firing a gun - once you've made the decision to put a bullet into the atmosphere, the consequences are on you.