In order to know the formation epoch of the oldest elliptical galaxies as a function of mass and observed redshift, a statistical analysis of 333 extremely red objects (EROs) classified as old galaxies (OGs) at 0.8 <= z <= 2.3 is carried out. Once we get MV 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 average 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.