Who makes the decisions concerning the guilt or innocence of a juvenile and why?

Juvenile Court Judge makes the decision.

Since there is no jury in the Juvenile Justice system, the Juvenile Court Judge makes the decisions regarding whether the juvenile is guilty or innocent.

Explanation:

The Juvenile Justice System is the court system that is established to handle the cases of juveniles (under 18 criminals). Juvenile cases are handled differently from those of adults. The primary purpose to rehabilitate the youth and restore him into the community.

There are no juries in the Juvenile Justice System, thus the cases are heard by Juvenile Court Judge and he decides after listening to the shreds of evidence of prosecutor, whether the juvenile is guilty or innocent.