Abstract Background and Aims The long-term outcomes of paediatric-onset inflammatory bowel disease pIBD in non-Caucasian populations are unknown. We therefore evaluated and compared the clinical features and long-term outcomes of pIBD with those of adult-onset IBD aIBD using a population-based cohort in the Songpa-Kangdong district of Seoul, Korea. Methods Clinical characteristics and prognoses were compared between the two groups: pIBD defined as <18 years of age at diagnosis and aIBD 18–59 years of age at diagnosis. Results We identified 131 patients with pIBD (48 ulcerative colitis UC, 83 Crohn’s disease CD) and 1192 patients with aIBD 866 UC, 326 CD during 1986–2015. Extensive colitis at diagnosis was more prevalent in pUC than in aUC 45.8% vs 22.3%, p < 0.001, and the overall exposure to corticosteroids, thiopurines and anti-tumour necrosis factor agents was higher in pUC than in aUC p < 0.001. The cumulative risk of colectomy was higher in pUC than in aUC during a median follow-up of 125.0 and 112.1 months, respectively 8.9% vs 1.8% at 10 years after diagnosis, p = 0.030. Ileocolonic location and inflammatory behaviour at diagnosis were more common in pCD than in aCD; however, patients with pCD and aCD did not differ regarding treatment or disease course during a median follow-up of 137.2 and 120.9 months, respectively. Conclusion Our study showed clear differences between pIBD and aIBD, especially in UC. pUC presents with more extensive diseases and may have a more severe disease course, as suggested by an earlier time to administering medications and performing colectomy.