Distribution of age- and metallicity-sensitive line strengths of our MUG sample. The error bars are shown at the 1σ level. The blue (red) circles show the results for galaxies with a Sérsic index n < 2.5 (> 2.5), with their sizes referring to their stellar masses (6−8 × 10¹⁰ M_⊙ are depicted by small circles while > 8 × 10¹⁰ M_⊙ are the big circles). For reference, we include as grey dots the measurements of the general sample of GAMA massive galaxies (> 10¹¹ M_⊙) with AAT/AAOmega spectra. The horizontal dashed line – at D_n(4000)=1.5 – splits the sample between old (above the line) and young populations. The green lines trace the evolution of a simple stellar population at solar metallicity (see text for details).