Comparison of mass-weighted stellar ages (in million years) calculated by CIGALE, with (blue shaded contours) and without (black line contours) using Hδ and D_n4000 in the fitting process, for sources in the reference catalogue (left panel) and AGNs (right panel). The addition of the two indices in the SED fitting provides additional constraints on the stellar population and allows CIGALE to calculate, on average, more meaningful stellar ages, in the sense that the distributions of the mass-weighted stellar age are flatter and better resemble the distribution of the age of the Universe, as compared to the highly peaked (at low values) and mass-weighted stellar age distributions without the spectral indices.