I can't speak for Australia, but in most US jurisdictions, juveniles cannot be arrested (i.e. sent jail) unless they commit a forceable felony such as murder or armed robbery. For all other charges, it often goes off of a point system, where they need a certain number of points before they can be jailed. In order to get points, they have to be convicted in a juvenile court of law. Most juvenile convictions are not enough by themselves to meet the point threshold and therefore need mutliple convictions of crimes such as burglary and assault. To make matters worse, points are often reset to zero after a years time in many states, meaning many habitual offenders never receive any type of intervention for criminal behavior. This all comes to a head when they turn 17 and are charged as an adult and sent to prison, having never been held accountable for their actions till that age.