Comparison between the mass-weighted ages obtained from simulation and those obtained from observations by considering the measurement errors. The green, red, and blue lines represent the samples of BCGs, MGs, and FGs, respectively. The thin dotted lines represent the age distributions obtained for different LRG samples by the full spectrum fitting in this paper, after adding a correction of −0.09 dex, and the thick solid lines represent the age distributions obtained from a semi-analytic galaxy formation model (Guo et al. 2011) by considering a scatter of 0.1 dex.