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 (∼10^{10} M⊙ ) are older (typically >∼1 Gyr old) and formed earlier than less massive galaxies in our sample.