This study tested several theoretically important differences between youth with a childhood-onset and youth with an adolescent-onset to their severe conduct problems. Seventy-eight pre-adjudicated adolescent boys (ranging in age from 11 to 18) housed in two short-term detention facilities and one outpatient program for youth at risk for involvement in the juvenile justice system participated in the current study. The sample was divided into those with a childhood-onset to their serious conduct problem behavior (n = 47) and those with an adolescent-onset (n = 31). The childhood-onset group showed greater levels of dysfunctional parenting, callous-unemotional traits, and affiliation with delinquent peers. The only variable more strongly associated with the adolescent-onset group was lower scores on a measure of traditionalism.