In an aging stellar population, the number fractions of chemically peculiar (CP) stars rise and fall periodically, and the fractions are high enough that their spectra features survive to appear in integrated light at subtle levels. Chemically peculiar stars are suggested to be used to disentangle an age history from integrated light spectroscopy. In this dissertation, we first introduce the improvement on stellar evolutionary isochrones and composite stellar population models. Individual elements C, N, O, Mg, Si, and Fe were included to gauge the effects on temperatures, luminosities, and lifetimes in various phases. Then, Integrated-light models that incorporate common types of chemically peculiar stars are assembled using synthetic spectra. CP spectra features are analyzed for local group galaxies and SDSS galaxies. Finally, we attempt to build an age technique with CP features. Four indices are defined from HgMn spectra features. Two age indicators are constructed and applied to SDSS galaxies for analysis of star formation history.