Our legal system provides a set of punishments for crimes and these punishments are carefully designed to be fitting for the level of crime. When someone, even a juvenile, commits a crime they deserve the prescribed punishments for their actions. Luckily, our justice system works very hard to determine innocence or guilt before setting punishments and so it is adequate to serve justice of juveniles.
Typically adequate justice is not served to juveniles guilty of horrible crimes. However in the case of the Massachusetts teen charged with rape and murder of his teacher in 2013, this teen was adequately served justice, as was the teacher's family. He will go to prison for a long time.
I definitely think that the US legal system serves adequate justice to juveniles. In the legal system, juveniles who truly are not old enough to understand what they've done wrong are treated as such, but those who are old enough to understand right and wrong get the justice they deserve.
The legal system in the U.S. does not always work for juveniles. When a juvenile commits a very adult crime, my gut is that they should be treated as an adult. But does this serve the child well? The human brain does not fully develop reasoning skills until a person is 25. A teen who commits a crime like rape and murder, do they really understand the enormity of their crime? Some people would say yes they do, others would say they do not. Here is where I feel that the justice system fails. If we put them away for years in an adult facility, this can cause more harm. Juvenile facilities may provide more services, but the child could be released at 18 and not be any different than when they went in. There is no easy answer to this question.