He wasn't an adult or a child--he was a teen, old enough to be tried for murder under our laws and to serve jail time. 

The typical child soldier lives in a war-torn country without our social services. The child is either orphaned or kidnapped--the very definition of helpless. Mr. Khadr, while not an adult at the time of his crimes, was yet old enough to accept some measure of responsibility for his actions--the measure of responsibility we expect from a teen charged with a serious crime.