In fairness, I don't think a youth masturbating should preclude him from working with kids later in life - if it did, there'd be very few men working with kids!
Men with convictions for violence, however, are a different matter, especially if kids were caught up in that violence. I know these days, anyone who wants to work with kids has to have a background check (I'm not sure about volunteering) - such a background check would have meant he wouldn't be allowed to work with kids - either the safeguards have changed or something slipped through the net.
I know of three instances of violence (involving weapons) prior to 2006, when his DNA was identified via the database check. I've never been able to discover what happened in 2006 that brought his DNA into this case, though. One of those instances involved trying to break down the door of a house with a young child inside, armed with an iron bar, screaming threats of serious assault at one of the adults in the house.