Research shows that those who have had obesity from childhood have greater risk of cardiometabolic disease compared to those who only develop obesity in adulthood. A main way of mitigating the risk of cardiometabolic disease in obesity is with weight loss. Weight loss has been shown to positively affect cardiorespiratory fitness (CRF), muscle strength, and body composition of adults. However, it is unclear whether the response of the aforementioned outcomes to weight loss may be influenced by age of obesity onset. The current study investigated how CRF, muscle strength, and body composition of adults with childhood- versus adulthood-onset obesity are affected by weight loss. Measurements were conducted before, at 12 weeks, and after ~10% weight loss. In total, 37 adults completed the 12-weeks assessments (age=30.5±3.2 y; BMI=32.9±3.0 kg/m2) and 23 participants with adulthood-onset (n=11) and childhood-onset obesity (n=12) reached 8.8±3.2% weight loss. The YMCA cycle ergometer test (YMCA) and the 20-m shuttle run test (20MSR) were used to measure CRF (ml.kg-1.min-1). A handgrip dynamometer was used to measure muscle strength. Body composition was assessed by DEXA and CT-scan. Overall, body composition and CRF improved (time effect: p<0.05) after 12 weeks and moderate weight loss and there was no group-by-time interaction for YMCA, 20MSR, muscle strength, and body composition variables. Age of onset of obesity does not affect the responsiveness of CRF, handgrip strength, and body composition at 12 weeks and after moderate weight loss in adults with obesity.