In order to know the formation epoch of the oldest elliptical galaxies as a function of mass and observed redshift, a statistical analysis for 333 extremely red objects (EROs) classified as old galaxies (OGs) at 0.8<z<2.3 is carried out. Once we get M_V and (B-V) at rest for each galaxy, we calculate the average variation of this intrinsic color with redshift and derive the average age through a synthesis model (the code for the calculation of the age has been made publicly available). The

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... ge gradient of the (B-V) color at rest of EROs/OGs is 0.07-0.10 Gyr^-1 for a fixed luminosity. The stars in these extremely red elliptical galaxies were formed when the Universe was 2 Gyr old on average. We have not found a significant enough dependence on the observed redshift and stellar mass: dt_formation/dt_observed=-0.46+/-0.32, dt_formation/(d log_10 M_*)=-0.81+/-0.98 Gyr. This fits a scenario in which the stellar formation of the objects that we denominate as EROs-OGs is more intense at higher redshifts, at which the stellar populations of the most massive galaxies form earlier than or at the same time as less massive galaxies.