  The study of the ages of early-type galaxies and their dependence on galaxy
mass and environment is crucial for understanding the formation and early
evolution of galaxies. We review recent works on the M/L ratio evolution, as
derived from an analysis of the Fundamental Plane of early-type galaxies at z~1
both in the field and in the clusters environment. We use the M/L ratio to
derive an estimate of the galaxy age. We also use a set of high-S/N
intermediate-resolution VLT spectra of a sample of early-type galaxies with
0.88<z<1.3 from the K20 survey to derive an independent estimate of their age
by fitting SSP model spectra. Taking advantage of the good leverage provided by
the high sample redshift, we analyse the results in comparison with the ages
obtained for the same sample from the analysis of the M/L ratio, and with the
predictions of the current hierarchical models of galaxy formation.
