To my knowledge, he was not arrested while a juvenile. I've never read that in any published documentation or heard of it in any retrospective documentaries or interviews. If you know of one such source, by all means I would be interested to hear of it.
When he was arrested for the first time, he was released on bail pending charges. About two months later, he was identified in a lineup by Carol DaRonch as the man who had tried to kidnap her. From then on, he was either incarcerated or a fugitive.
The point of this is to illustrate that many killers, including serial killers, had no trouble or altercation with law enforcement throughout the trajectory of their escalating crimes and violent behavior. They were, to borrow the phrase, the last person in the world people would have suspected. So it is not outside the realm of possibility that the offender in Delphi is similarly regarded as a perfectly normal, law-abiding, non-threatening person with no record of criminal or violent behavior.