  Observations of large amount of massive galaxies with relatively old
populations found at high redshifts are challenging galaxy formation scenarios
within standard cosmology. Precise determinations of the average age of these
galaxies would be useful for the discussion of this problem. Here we carry out
a better constraint of the age of 200 V-shaped SED non-AGN galaxies at
redshifts $2<z<4$ of the catalog of FourStar Galaxy Evolution Survey,
identified by V-shape in their spectral energy distribution (SED) with a Lyman
and a Balmer break. SED fitting include a main stellar population in addition
to a residual younger population and extinction. The galaxies are younger at
higher redshift on average. However, for the galaxies with $z>2.5$, we do not
see a significant evolution of their average age, with all average ages of
galaxies mostly remaining between 1 and 2 Gyr. Our research find that most
massive galaxies ($\sim 10^{10} M_\odot$ ) are older (typically $>\sim 1$ Gyr
old) and formed earlier than less massive galaxies in our sample.
