  In a sample of about 45,700 early-type galaxies extracted from SDSS, we find
that the shape, normalization, and dispersion around the mean size-stellar mass
relation is the same for young and old systems, provided the stellar mass is
greater than 3*10^10 Msun. This is difficult to reproduce in pure passive
evolution models, which generically predict older galaxies to be much more
compact than younger ones of the same stellar mass. However, this aspect of our
measurements is well reproduced by hierarchical models of galaxy formation.
Whereas the models predict more compact galaxies at high redshifts, subsequent
minor, dry mergers increase the sizes of the more massive objects, resulting in
a flat size-age relation at the present time. At lower masses, the models
predict that mergers are less frequent, so that the expected anti-correlation
between age and size is not completely erased. This is in good agreement with
our data: below 3*10^10 Msun, the effective radius R_e is a factor of ~2 lower
for older galaxies. These successes of the models are offset by the fact that
the predicted sizes have other serious problems, which we discuss.
