Although many studies have examined the relationship between early deviant behavior and subsequent life problems among adolescents drawn from the general population, such relationships have not been examined for youth attending substance abuse treatment. Based on in-depth psychosocial assessments conducted with adolescents entering an outpatient substance abuse treatment program (N = 193), the current study examines individual characteristics, life circumstances, and other behavioral and psychological characteristics that are correlated with the age at which these youth initiated substance use and criminal activity. Early onset of substance use was associated with greater levels of family deviance and a variety of problems including school adjustment, drug use, criminal involvement, bullying and cruelty to people and animals, and involvement in risky sexual activities. In contrast, early onset of crime was related only to male gender, early onset of substance use, and cruelty to people. Findings suggest that treatment providers may need to consider the ages at which their adolescent clients initiated substance use given its association with illegal activity, other deviant behavior, and precocious and high-risk sexual behaviors.