Fraction of old galaxies f_old as a function of absolute magnitude M_r for the SDSS galaxy sample at S/N ≥ 10. Galaxies are classified as old based on whether they lie above the dividing line corresponding to the minimum of the D4000_n distribution (Fig. 8c), making this classification dependent on aperture corrections. Trends based on uncorrected values (dark red line) and aperture-corrected values (black line) are shown. The vertical arrows denote the transition magnitudes for each dataset, marking the luminosity above which old galaxies predominate.