at a time $t_1$ between redshift $0 < z < 1.5$ with uniform probability for $t_1$. For simplicity, we use discrete values for the ages with spacing 0.1 Gyr and assume a flat $\Omega_m=0.27$, $w=-1$ and $h=0.71$ cosmology to convert time to redshift. The metallicity for a given galaxy is chosen to be uniformly distributed in the range $0.1 <
Z/Z_{\odot} < 4$. Finally, we add random noise to the spectra so that $S/N=15$ and determine the age with the same procedure used for the data.

The results of our Monte Carlo simulations are shown in Figure 5. The top panel shows the simulated age–redshift distribution if only six early-types are found per redshift interval[^3]. The edge is blurred and smeared due to recent star formation contamination and because of the error in the age determination; nevertheless, it is clearly recognizable. The galaxies observed within a few 100 Myr of bursts have very small single-burst equivalent ages, as observed for some galaxies in the LRG and Treu et al. sample, but they do not interfere with the location of the old envelope.

The lower panel shows the distribution for the case where 60 early-types are observed per redshift bin (this 