Objective This study aimed to assess the performance of a commonly used ICU severity score (SAPS3) and determine whether an alternative scoring system may be more accurate across all age strata. Methods Retrospective cohort study in a general ICU in Brazil. A secondary analysis was performed with clinical and epidemiological data, present in the first 24 hours of unit admission. Then, a binary logistic regression, followed by cross-validation, was made to develop a novel prognostic tool. ICU mortality was the primary outcome evaluated. Results A total of 3042 patients were included over the study period between August 2015 and July 2018 with a median age of 67 ± 18.4 years. SAPS3 performed fairly in prediction of ICU mortality, particularly in the 80 years or older subset. Multivariable regression identified variables independently associated with mortality that were used to develop the Age Calibrated ICU Score (ACIS) tool that performed similarly to SAPS3 across age categories, being slightly superior in the very elderly population (AUC 0.80 vs 0.72). Conclusions The ACIS offers a robust and simple tool to predict ICU mortality, particularly in an increasingly elderly critical care population.