BACKGROUND: Gambling is a prevalent behavior, yet few studies have investigated its mental health correlates. Although early-onset engagement in behaviors with addictive potential has generally been associated with more severe problems, direct investigation of a nationally representative sample of gamblers grouped by age at onset of gambling has not been performed. OBJECTIVE: To identify differences in psychiatric correlates of gambling and gambling-related attitudes and behaviors in adolescents (aged 16-17 years) and in young adults (aged 18-29 years) with early-onset (before age 18 years) and adult-onset gambling. DESIGN: Logistic regression analysis. SETTING: Public access data set derived from random-digit-dialing telephone surveys. PATIENTS: The study analyzed data from adolescent (n = 235), early-onset adult (n = 151), and adult-onset (n = 204) past-year gamblers and adolescent (n = 299) and adult (n = 187) nongamblers in the Gambling Impact and Behavior Study. MAIN OUTCOME MEASURES: Gamblers and nongamblers were compared within each group on measures of sociodemographics and psychiatric health. Adolescent, early-onset adult, and adult-onset past-year gamblers were compared on measures of gambling attitudes and behaviors. RESULTS: Adolescent gamblers were more likely than adolescent nongamblers to report alcohol and drug use and abuse/dependence and depression. Elevated rates of alcohol and drug use and abuse/dependence were observed in early-onset adult gamblers vs adult nongamblers, and only elevated rates of alcohol use were observed in adult-onset gamblers vs adult nongamblers. Substantial differences in reasons for and patterns of gambling were observed among the 3 groups of gamblers. CONCLUSIONS: Adolescent-onset gambling is associated with more severe psychiatric problems, particularly substance use disorders, in adolescents and young adults. More research is needed to investigate the relationships and inform prevention and treatment strategies.