Much of the phosphorus (P) in grain-based concentrates fed to growing horses is in the form of phytate-P. Little is known about the ability of young horses to degrade phytate-P or whether age affects mineral digestion in horses. The objective of this study was to examine the effect of age on P, calcium (Ca), and magnesium (Mg) digestibility and phytate-P degradation. Four long yearling geldings and 4 mature geldings were fed a diet of alfalfa cubes, timothy cubes and a pelleted concentrate. The diet contained 0.28% total P and 17.4% of that P was in the phytate form. There was a 14-d diet adaptation period followed by a 4-d fecal collection period. There was no difference in apparent P digestibility between the 2 age groups (P > 0.05). Phytate-P disappearance was 94.8% and did not differ between ages (P > 0.05). Apparent Ca digestibility tended to be higher in mature geldings (P = 0.0526), but apparent Mg digestibility did not differ between ages (P > 0.05). Long yearlings have the same ability to digest P and Mg as mature geldings, but more research is needed to determine the effect of age on Ca digestibilities.