Objective: The incidence and prevalence of granulomatosis with polyangiitis (GPA) in the US is not well characterized. Owing to its rarity, outcomes data in pediatric-onset GPA are also lacking. The aims of this study were to describe the epidemiology of GPA and outcomes in GPA patients in the US, and to compare outcomes between pediatric and working-age adult patients. Methods: A retrospective cohort study using the 2006–2014 Truven Health Analytics MarketScan Commercial Claims and Encounters Database was conducted. The incidence and prevalence rates of pediatric and adult GPA (age <65 years) were calculated. Outcomes among the 2 age groups were analyzed. Results: A total of 5,562 cases of GPA were identified, of which 214 (3.8%) were pediatric onset and 5,348 (96.2%) were adult onset. The incidence rate of pediatric-onset GPA was 1.8 cases per 1 million person-years, compared to 12.8 cases per 1 million person-years in working-age adults. There was a slight female preponderance in both groups (63% and 53% among pediatric and adult GPA patients, respectively). Rates of hospitalization and severe infections were high in both children and working-age adults, but children had more frequent hospitalizations (rate ratio [RR] 1.3 [95% confidence interval (95% CI) 1.1–1.4]) and 2–3-times higher rates of leukopenia (RR 2.6 [95% CI 1.5–4.3]), neutropenia (RR 2.2 [95% CI 1.2–4.0]), and hypogammaglobulinemia (RR 3.7 [95% CI 2.0–6.4]). Time-to-event analyses showed no differences in the time to hospitalization, severe infection, major relapse, or end-stage renal disease. Conclusion: This study represents the largest cohort of GPA reported to date. Pediatric GPA patients experienced more frequent hospitalizations and were more vulnerable to hematologic complications than non-elderly adult patients.