Cardiorespiratory fitness (CRF) is a strong predictor of morbidity and mortality in patients with obesity. This study investigates the CRF range and its clinical determinants in patients with obesity. Moreover, a practical proposal for CRF interpretation is provided. In this study, 542 patients (69% females) with BMI ≥ 30 kg/m2 performed an incremental cardiopulmonary exercise test (CPET). Patients had a median (IQR) age of 47.0 (6.2) years with a mean BMI of 41.7 ± 6.7 kg/m2. Normal values curves of VO2peak/kg showed a median (IQR) of 20.3 (37.6) mL/min/kg. The lower-quartile threshold of VO2peak/kg was at 17.9 mL/min/kg. Analysis of covariance revealed that VO2peak/kg inversely correlates with age and BMI with a significant age × BMI interaction effect (all p < 0.0001); as BMI class increases, CRF decreases, but a smaller age-related decline in VO2peak/kg is observed. A multivariate logistic regression demonstrated that belonging to the lower quartile of VO2peak/kg was independently determined by age (OR 2.549, 95% CI 1.205–5.392, p < 0.0001) and BMI (OR 5.864, 95% CI 2.920–11.778, p < 0.0001) but not by comorbidities. At very high BMI, the effect of age on functional capacity is lower, suggesting that BMI acts as an “aging factor” on CRF. Age and BMI, but not comorbidities, are independent determinants of low VO2peak/kg.