When's the last time you've heard of a 7 year old committing murder?

I seem to recall ( a bit fuzzy, but I'm quite drunk right now) 2 nine or ten year olds in the UK abducting and murdering a todller?

Yes, that was a shocking case. And it's pretty clear that his killers were badly mentally disturbed in such a way as to present genuine danger to the public -- just as with an adult who kills for fun. Having a mental age of seven, or ten, or thirtysomething, or a chronological age of ten, is a side issue really. In the UK we don't have the death penalty for murder, but I expect if we did that those two would have got it. Keeping people who kill for fun off the streets is the issue here; unless you allow for the possibility of reform, which some clearly do. Perhaps the answer is that if someone is ten, they'll one day have the mental age of an adult, i.e. they could still learn; an adult with the mental age of seven doesn't usually present that possibility.

Peter