  (Abridged) We study the stellar populations of 14 elliptical galaxies in the
Virgo cluster. We propose an alternative approach to the standard side-band
method to measure equivalent widths (EWs). Our Boosted Median Continuum maps
the EWs more robustly than the side-band method, minimising the effect from
neighbouring absorption lines and reducing the age-metallicity degeneracy. We
concentrate on Balmer lines (Hbeta,Hgamma,Hdelta), the G band and the 4000A
break as age-sensitive indicators, and on the combination [MgFe] as the main
metallicity indicator. We go beyond the standard comparison of the observations
with simple stellar populations (SSP) and consider various models to describe
the star formation histories, either with a continuous star formation rate or
with a mixture of two different SSPs. Composite models are found to give more
consistent fits among individual line strengths and agree with an independent
estimate using the spectral energy distribution. Our age and metallicity
estimates correlate well with stellar mass or velocity dispersion, with a
significant threshold around 5E10 Msun above which galaxies are uniformly old
and metal rich. In a more speculative way, our models suggest that it is
formation **epoch** and not formation timescale what drives the Mass-Age
relationship of elliptical galaxies.
