Using deep Hubble Space Telescope Advanced Camera for Surveys imaging and Very Large Telescope FOcal Reducer/low dispersion Spectrograph 2 spectra, we determined the velocity dispersions, effective radii, and surface brightnesses for four early-type galaxies in the z = 1.237 cluster RDCS 1252.9-2927. All four galaxies are massive, greater than 1011 M☉. These four galaxies, combined with three from RDCS 0848+4453 at z = 1.276, establish the fundamental plane of massive early-type cluster galaxies at = 1.25. The offset of the fundamental plane shows that the luminosity evolution in rest-frame B is Δ ln = (-0.98 ± 0.06)z for galaxies with M > 1011.5 M☉. To reproduce the observed mass-to-light ratio (M/L) evolution, we determine the characteristic age of the stars in these M > 1011.5 M☉ galaxies to be 3.0 Gyr; i.e., z* = 3.4. Including selection effects caused by morphological bias (the "progenitor bias"), we estimate an age of 2.1 Gyr, or z* = 2.3 for the elliptical galaxy population. Massive cluster early-type galaxies appear to have a large fraction of stars that formed early in the history of the universe. However, there is a large scatter in the derived M/L values, which is confirmed by the spread in the galaxies' colors. Two lower mass galaxies in our = 1.25 sample have much lower M/L values, implying significant star formation close to the epoch of observation. Thus, even in the centers of massive clusters, there appears to have been significant star formation in some massive, M 1011 M☉, galaxies at z 1.5.