This study examined the association between age of onset and persistence of externalizing behavior and young adults' cannabis use disorders (CUDs). Data were from a 21 year follow-up of a birth cohort study in Brisbane, Australia. The present cohort consisted of 2225 young adults who had data available about CUDs at 21 years and externalizing behavior at 5 and 14 years. Young adults' CUDs were assessed using the CIDI-Auto. Child and adolescent externalizing behavior were assessed at the 5- and 14-year phases of the study. After controlling for confounding variables, children who had externalizing behavior at both 5 and 14 years (child-onset-persistent) (COP) had a substantial increase in risk of CUD at age 21 years (Odds ratio (OR) = 2.5; 95% CI: 1.5, 4.2). This association was similar for those who had 'adolescent onset' (AO) externalizing behavior. However, there was no association between 'childhood limited' (CL) externalizing behavior and CUD. Externalizing behavior in adolescence is a strong predictor of subsequent CUD. Smoking and drinking at 14 years partially mediated the link between externalizing behavior and CUD.