Objectives: This study examined differences in the clinical and treatment-related features of pathological gambling (PG) on the basis of the age of PG onset among pathological gamblers who sought treatment. Methods: A total of 702 male outpatients with a primary diagnosis of PG and who were treated in a clinical practice were assessed by retrospective chart review. We selected the age of 25 years and younger as the threshold for "group 1."We then stratified the participants into 4 groups on the basis of the age of PG onset in 10-year intervals. Analysis of covariance with a covariant of age and the Pearson chi2 test were used for analyses. Results: We found that the earlier-onset gamblers were less likely to be escape type (P < 0.05), used significantly more Internet-based gambling (P < 0.001), and were less likely to engage in nonstrategic gambling (P < 0.05) than the later-onset gamblers. In addition, the earlier-onset gamblers took anticraving medication, such as naltrexone, significantly more often (P < 0.05), and sought treatment significantly more slowly after the onset of PG than the later-onset group (P < 0.01). Regarding adherence to treatment, however, there was no significant difference among the 4 groups on the basis of the age of PG onset. Conclusions: The age of PG onset is associated with several important clinical and treatment features. More studies are needed to advance prevention and treatment strategies for each age group.