Left panel: mean stellar population ages of MPGs as derived from the SED fitting as a function of redshift and mean stellar mass density for high- and low-Σ MPGs (filled points). The colour and symbols code is the same as in Fig. 6. Error bars indicate the error on the mean. Points refer to the redshift bins 0.5z<0.7,0.7z<0.9, 0.9z1.0. The arrows track the increase of the ages in case of passive evolution from z = 0.95 to z = 0.6. Right panel: filled points: the mean values of D4000_n directly measured on spectra for MPGs with different Σ (symbol and color code the same as in the left panel). Open dark red triangles indicate the values of D4000_n of high-Σ MPGs derived from a BC03 model with age/τ/Z equal to the mean values derived from the SED fitting of this sub-population (D4000_n,SED). Open green squares are the equivalent for low-Σ MPGs, once the aperture bias is taken into account. Actually, for high-Σ MPGs the slit aperture samples approximately the whole galaxy (see Fig. 8), thus D4000_n measured directly from the spectra and D4000_n,SED can be fairly compared. For low-Σ MPGs Fig. 8 shows that the slit samples just the region within 0.5 R_e. In the comparison of D4000_n and D4000_n,SED hence, we have to take into account the effect of colour gradients (see more details in Sect. 5.2). Open green circles indicate the values of D4000_n,SED of low-Σ without any correction for aperture bias.