Nonlinear association between age and 28-day mortality. A, The model is unadjusted. Age-linear akaike information criterion (AIC): 30,713 versus age-spline AIC: 30,686, likelihood ratio test p value for AIC difference: < 0.001. B, This model is further adjusted for demographic characteristics, including male sex, and presence of hypertension, diabetes mellitus, coronary artery disease, congestive heart failure, chronic obstructive pulmonary disease, and active cancer. Age-linear AIC: 30,218 versus Age-spline AIC: 30,160, likelihood ratio test p value for AIC difference: < 0.001. C, This model is further adjusted for acute ICU physiologic factors, including symptom onset less than or equal to 3 d prior to ICU admission, lymphocyte count less than 1,000/uL, degree of hypoxemia and respiratory support, shock, SOFA coagulation greater than 0, SOFA liver greater than 0, and SOFA renal greater than 0, and the number of ICU beds. Age-linear AIC 26,913 versus Age-spline AIC: 26,783, likelihood ratio test p value for AIC difference: < 0.001. D, This model is further adjusted for COVID-19 treatments, including: remdesivir, tocilizumab, and corticosteroids. Age-linear AIC: 26,778, Age-spline AIC 26,643, likelihood ratio test p value for AIC difference: < 0.001. SOFA = Sequential Organ Failure Assessment.