Fat oxidation decreases age, yet, no studies have previously investigated if aging affects the maximal fat oxidation rate (MFO) during exercise in men and women differently. We hypothesized that increased age would be associated with a decline in MFO and this would be more pronounced in women due to the menopause, compared to men. In this cross-sectional study design, 435 (247/188, male /female) subjects of varying ages performed a DXA-scan, a submaximal graded exercise test and a V̇O2max test, to measure MFO and cardiorespiratory fitness (CRF) by indirect calorimetry. Subjects were stratified into 12 groups according to sex (M/F), age (55 years), CRF (below average (BA) and above average (AA)). Women 55 years. In summary, we found that women <45 years display a higher MFO (mg/min/kg FFM) compared to men and that this sexual divergence is abolished after the age of 45 years. Novelty: • Maximal fat oxidation rate is higher in young women compared to men • This sex related difference is attenuated after the age of 45 years • Cardiorespiratory fitness does not influence this sex related difference.