We regularly find the mentally disabled innocent on the basis of limited understanding. So the distinction is already part of the adult justice system. I think the distinction between the two separate sentencing systems is really not about the defendant's developmental level but rather how much we expect the person to develop beyond their current moral status.
An adult might be found innocent based on mental age, but would still not be tried as a juvenile on that basis, because juvenile facilities and procedures are meant to accommodate the child's ongoing development and to really educate, in the sense of 'shaping personality' rather than just to rehabilitate. The adult is not expected to be subject to the same kind of shaping.
Of course, to some degree this is provably incorrect, people do continue to develop, and prisoners can find something in the experience that is less of a rehabilitation of their viewpoint and more of a new dimension of their personality. (Tim Robbins' drama projects, for instance, seem to work in this way -- by giving the participants a different kind of theory of mind and empathy than they developed the first time around through having them play roles for extended periods.) But it is not the expected response of an adult to punishment.
To try a child as an adult, you basically have to make the case they will not grow out of their criminal mindset. This can be because the crime indicates psychopathology that does not tend to be treatable, or because the child has so little time left before growth is complete that they are unlikely to come that far by then. So folks that usually get charged as adults are at least 16.
My guess is that this case must fall in the first category, or no one would dare charge someone so young as an adult. Something in the child's method must indicate he is depraved, and not just impulsive.