Early onset of substance use has demonstrated a robust relationship with later severity of use, yet this relationship has been less examined in criminal justice contexts or examined differentially by gender. The present study utilized secondary data from a sample of offenders in one small Midwestern state entering corrections-based substance abuse treatment in prison, jail, or community settings to address these gaps in the literature. Findings indicated that age of regular cigarette use did not uniquely affect severity of illicit drug use in adulthood, defined as the frequency of use or endorsement of substance use disorder criteria. In comparison, ages of onset for alcohol and illicit drug use were significantly associated with multiple severity variables examined. Gender was not found to moderate these relationships. These findings emphasize that age of use onset for alcohol and illicit drugs can be valuable in identifying offenders at risk of severe substance use in adulthood.