The fat oxidation capacity is higher in young compared to elderly subjects and higher in premenopausal compared to postmenopausal women, but the influence of age on maximal fat oxidation (MFO) is not clear. Therefore, this study aimed to evaluate MFO (g/min) across the lifespan of trained adult women. In total, 36 healthy trained women were recruited into three groups: (n = 12), young (27 ± 3 years, mean ± SD) premenopausal, middle-aged (57 ± 3 years), and older (71 ± 2 years) postmenopausal women and all had a body mass index <25 kg/m². After an overnight fast, body composition was determined by dual-energy X-ray absorptiometry, and blood samples were obtained. A FAT_max-test was performed on a cycle ergometer, and MFO was calculated from the pulmonary V̇O₂ and V̇CO₂ measured by indirect calorimetry. The absolute MFO was significantly higher in young (0.40 ± 0.07 g/min) compared to both middle-aged (0.33 ± 0.07 g/min) (p = 0.035) and old (0.25 ± 0.05 g/min) women (p < 0.001). Absolute MFO was higher in middle-aged compared to old women (p = 0.018). Relative MFO (MFO/LBM, mg/min/LBM) was higher in young (8.39 ± 1.62 mg/min/LBM) compared to old (6.16 ± 1.14 mg/min/LBM) women (p = 0.004). A significant linear relationship was observed between absolute MFO and age (R² = 0.41; p < 0.001), V̇O_2max (R² = 0.40; p < 0.001), and LBM (R² = 0.13; p = 0.033), respectively, and between relative MFO and fat mass (R² = 0.12; p = 0.04). In conclusion, the maximal capacity to oxidize fat is attenuated with age in trained women. Furthermore, postmenopausal middle-aged women have higher absolute MFO compared to older women, and this implies that it is age per se and not a change in estrogen availability that leads to lower absolute MFO.