The aging behavior of a 1.5Y-TZP with 0.25 wt% of alumina exhibiting high mechanical properties was investigated in both water vapor and in air with in situ XRD analysis. The results showed that the material exhibited a high resistance to aging in water vapor at 134 °C compared to 3Y-TZP, which is explained by the addition of alumina and a lower sintering temperature. The aging kinetics were fitted by the Mehl–Avrami–Johnson law, three stages were identified corresponding to predominance of nucleation mechanism for temperatures below 180 °C and above 340 °C and to growth mechanism for intermediate temperatures. It was shown that for the temperature range investigated in water vapor and air, the mechanism was independent on the environment. A strong influence of the cooling rate on the tetragonal to monoclinic phase transformation was observed and discussed using a time–temperature-transformation diagram derived from the aging results in air.