Obesity is a potentially modifiable risk factor for inflammatory bowel disease (IBD). We aimed to evaluate the body mass index (BMI) of those diagnosed with IBD early versus late in life in the context of age-adjusted background population. Patients with a new diagnosis of IBD from 2000 to 2021 were included. Early-onset IBD was classified as age <18 and late-onset IBD classified as age ≥65. Obesity was classified as BMI ≥30 kg/m2. Population data were obtained from community surveys. Included were 1573 patients (56.0%) with Crohn's disease (CD) and 1234 (44.0%) with ulcerative colitis (UC). Overall, the median BMI at IBD diagnosis was 20 kg/m2 (IQR 18-24) among those diagnosed at age <18 vs 26.9 kg/m2 (IQR 23.1-30.0) among those diagnosed at age ≥65 (rank-sum p<0.01). In all age groups, BMI was stable during the 1-year preceding IBD diagnosis. At age <18, 11.5% of the background population was obese compared with 3.8% of those with newly diagnosed CD (p<0.01) and 4.8% of those with newly diagnosed UC (p=0.05). At age ≥65, 23.6% of the population was obese compared with 24.3% of those with newly diagnosed CD (p=0.78) and 29.5% of those with newly diagnosed UC (p=0.01). Patients with IBD diagnosed at age <18 were less likely to be obese compared with the age-adjusted background population whereas those diagnosed at age ≥65 were more likely to be obese. Future prospective studies should investigate obesity as a modifiable risk factor for late-life IBD.