Background: People with young-onset obesity (diagnosed before age 25) may present with more cardiometabolic abnormalities and obesity-related complications and may be less likely to respond optimally to pharmacotherapy. Tirzepatide (TZP) is a once weekly GIP/GLP-1 receptor agonist approved for the treatment of type 2 diabetes (T2D) and under investigation for chronic weight management. We compared baseline characteristics and body weight (BW) change with TZP in participants with young vs later-onset obesity in SURMOUNT-1 which included adults with obesity/overweight with weight-related comorbidities without T2D. Methods: Post hoc analysis included participants (TZP/placebo pooled) with young (N = 946) vs later-onset obesity (N = 1560) with onset age derived from self-reported diagnosis date and birth date at study enrollment. Comparison of baseline characteristics was based on ANCOVA model and change from baseline in BW (placebo-adjusted, TZP pooled) at Week 72 based on MM RM. Results: Participants with young- versus later-onset obesity (mean age 37 vs 50 years) had longer duration of obesity (20 vs 11 years), higher BMI (40.4 vs 36.6 kg/m2), waist circumference (118 vs 112 cm), eGFR (106 vs 93 mL/min/1.73m2), alanine aminotransferase (28 vs 27 IU/L), and lower HbAlc (5.5 vs 5.6%), systolic blood pressure (121 vs 125 mmHg) and cholesterol (186 vs 196 mg/dL) at baseline, respectively (all p < 0.001). Participants with young-onset obesity achieved significantly greater BW loss with TZP than those with later-onset obesity (-20.8 vs -16.7 kg p < 0.001; -18.7 vs -16.8% p = 0.061). Conclusions: Participants with young-onset obesity demonstrated a mixed cardiometabolic risk profile, with some variables worse and some better than those with later-onset obesity. TZP led to substantial BW reduction irrespective of age of obesity diagnosis with greater BW loss in young- versus later-onset obesity. Future studies are warranted to better understand if early intervention in people with young-onset obesity may improve long-term outcomes.