What was your point?
At some stage, a child becomes capable of making adult decisions. 16, 18, 21, whatever. Once a date is set in law then anyone under that age must be tried as a child because it has been agreed, presumably with contributions from all the experts in child development, legal precedent and any other field that is relevant, that that is the minimum age at which the person can be held legally liable for the decisions they make. That's why it's commonly known as the "Age of Responsibility"
Bending those carefully crafted rules for vengeance's sake is morally wrong. The crime is irrelevant, it's the age/responsibility capability that matters.
you are half right age has nothing to do with it, it is the "responsibility capability" that matters.