This question actually contains two questions. Firstly, let us examine the criminalization issue. Research has demonstrated repeatedly that prisons, while serving as a means of punishing lawbreakers, unfortunately serve as an educational system for many incarcerated. Older, more experienced criminals often serve as teachers and tutors to younger prisoners in various criminal arts and in many instances these newly acquired skills are used by prisoners once released. This is one reason why many courts are hesitant to put minors into major prison populations instead of juvenile centers, even for serious crimes.
Now, let us look at the issue of the physical age of the defendant, the person's psychological emotional age, the environment in which the defendant was reared and the circumstances surrounding the criminal event. All of these factors contribute to the decision of the court as to whether to try the minor age individual as an adult or a minor. Did the minor know the difference between right and wrong? Did the individual commit the offense as a result of extenuating circumstances (rage or hatred resulting from long term abuse, etc.). Was the individual mentally or emotionally unbalanced when the act was committed? There is not a "one size fits all" sentencing approach used by the courts and these cases, because of the factors mentioned and others, are very complex because while society demands punishment for crimes, the courts must be careful not to punish in excess. The courts must be subjective in their approach.