(D_n4000, Hδ) diagnostics color-coded according to the difference between r- and u-band light-weighted ages for all models of the StePS-like spectral library (left panel), and color-coded according to rms in Δ(age) (right panel). We superimposed typical 1σ average uncertainties in measuring spectral indices in this plane at S/N = 10 (black error bars) and S/N = 30 (red error bars), respectively.