Age comparison with scaled ([M/H]) and unscaled ([Fe/H]) metallicities, from anNN trained on the BaSTI grid. Top panel: Age from scaled metallicity τ([M/H]) versus age from unscaled metallicity τ([Fe/H]). The number of stars in each sample is annotated in the upper panel corners, and running medians and ±σ lines are shown in solid and dotted white lines, as well as a one-to-one black dashed line to guide the eye. Bottom panel: Running median of the difference Δτ between the two age estimates in solid lines, and associated standard deviation in shaded areas.