Can you do that - make one crime not applicable to an age limit and all the others applicable?

You can treat each crime on its own merits, but I say again, I was referring to this one, not the judicial system in general.

If someone is a dangerous nutter the last thing you do is bang him up with hardened criminals - he is hospitalised and attempts are made to cure him.

I agree.

Presumably these children have been mentally assessed and declared sane enough to walk among us - if not, and they are a danger to the public then presumably they would have been sanctioned.

It seems that process didn't work. We pay 'experts' handsomely to make these decisions, and in this case they were wrong. Normally their failures are not made public.