The study defines the phenotypic characteristics of childhood-onset inflammatory bowel disease (IBD), comparing it with adult-onset IBD. It analyzed 416 children with childhood-onset IBD (276 Crohn's disease [CD], 99 ulcerative colitis [UC], 41 IBD type unclassified [IBDU]) and 1297 adults with adult-onset IBD (596 CD, 701 UC) using the Montreal classification. Childhood-onset IBD was characterized by a "panenteric" phenotype (ileocolonic plus upper GI tract involvement) with less isolated ileal or colonic disease compared to adults. UC was more extensive in children at diagnosis than in adults. Children with CD and UC required immunomodulatory therapy within 12 months of diagnosis. The median time to first surgery was longer in childhood-onset CD than in adult-onset CD. The study found that childhood-onset IBD is characterized by extensive intestinal involvement and rapid early progression. The findings suggest that childhood-onset IBD has a distinct phenotype, with more extensive disease and a higher likelihood of complications. The study also highlights the importance of using the Montreal classification to accurately classify and understand the phenotypic characteristics of IBD in children. The study provides new data on the evolution of IBD in children, including the need for immunomodulation and surgery. The results emphasize the importance of early diagnosis and treatment in childhood-onset IBD to prevent complications and improve outcomes. The study also highlights the need for further research to better understand the differences between childhood-onset and adult-onset IBD.The study defines the phenotypic characteristics of childhood-onset inflammatory bowel disease (IBD), comparing it with adult-onset IBD. It analyzed 416 children with childhood-onset IBD (276 Crohn's disease [CD], 99 ulcerative colitis [UC], 41 IBD type unclassified [IBDU]) and 1297 adults with adult-onset IBD (596 CD, 701 UC) using the Montreal classification. Childhood-onset IBD was characterized by a "panenteric" phenotype (ileocolonic plus upper GI tract involvement) with less isolated ileal or colonic disease compared to adults. UC was more extensive in children at diagnosis than in adults. Children with CD and UC required immunomodulatory therapy within 12 months of diagnosis. The median time to first surgery was longer in childhood-onset CD than in adult-onset CD. The study found that childhood-onset IBD is characterized by extensive intestinal involvement and rapid early progression. The findings suggest that childhood-onset IBD has a distinct phenotype, with more extensive disease and a higher likelihood of complications. The study also highlights the importance of using the Montreal classification to accurately classify and understand the phenotypic characteristics of IBD in children. The study provides new data on the evolution of IBD in children, including the need for immunomodulation and surgery. The results emphasize the importance of early diagnosis and treatment in childhood-onset IBD to prevent complications and improve outcomes. The study also highlights the need for further research to better understand the differences between childhood-onset and adult-onset IBD.