have a $S/N$ of 8. We also add the red radio galaxies 53W091 and 53W069 for which accurate ages have already been determined [@D+96; @Spinrad+97; @NDJH03 Dey et al., in preparation]. Examples of the spectra used are given in Figure 3.

The age of the dominant stellar population in the galaxies is obtained by fitting single stellar population models [@JPMH98; @JDMPP03] to the observed spectrum. These models have been extensively tested [@KFJ02; @JFMG03] and have two free parameters: age and metallicity. By construction, star formation is assumed to occur in a single burst of duration much shorter ($< 10$%) than the current age of the galaxy [@JFDTPN99]. Therefore, the derived ages are single-burst equivalent ages. The best fit model is found by standard $\chi^2$ minimization. Then, stellar ages and their errors are obtained by marginalizing the resulting likelihood surface over metallicity. Some examples for the parameter confidence regions, at the 1 and 2$\sigma$ level, are shown in Figure 3. Note that for galaxies with high $S/N$, like the Treu et al. sample, the constraints in both age and metallicity are very tight and thus the so-called age-metallicity degeneracy is dramatically reduced and does not affect the age determination at a 