Number density of galaxies for the SDSS DR7 in the mass-age and mass-metallicity planes, obtained using both the volume and completeness statistical weights. Left and central panels: Distributions obtained with light-weighted mean ages and metallicities, without the implementation of aperture corrections (left panels) and implementing aperture corrections (central panels). Right panels: Distributions obtained with mass-weighted mean ages and metallicities for the aperture-corrected sample. In the mass-age planes, the coloured black, green, and fuchsia lines identify the density levels enclosing 16%, 50%, 84%, and 97.5% of the complete galaxy sample, obtained by smoothing the corresponding distribution using a Gaussian KDE. The red and blue lines, overplotted on the top of the mass-age distributions in the left and central panels, for uncorrected and aperture-corrected estimates, respectively, identify the divisions between young and old galaxies, as defined in Sect. 4.2. In the mass-metallicity planes, the coloured black and green solid and dashed lines identify the median and 16^th and 84^th percentiles of the corresponding distributions in bins of mass. The red lines overplotted on top of the mass-metallicity distribution identify the median relation from Gallazzi et al. (2005). The dotted horizontal lines indicate the solar metallicity of Z_⊙ ≡ 0.02.