We aimed to explore the association between age of onset of gambling problems and current psychopathological and clinical status, personality profile and therapeutic outcome in a sample of pathological gamblers. A total of 904 consecutive pathological gambling patients were administered several instruments about gambling behavior, psychopathology and personality. They received a 4-month cognitive-behavioral group treatment. Information of dropouts and relapses during treatment was registered. Older age of onset of gambling problems was associated with higher general psychopathology (SCL-90-R Paranoid Ideation, Psychoticism, Depression; P < 0.015). Younger age of onset was related to greater severity of pathological gambling (P < 0.015), higher novelty seeking, and lower self-directedness (P < 0.015). No statistically significant association was found between age of onset and relapse and dropouts during treatment. Age of onset of gambling problems seems to influence the clinical presentation of pathological gambling but not treatment outcome.