True vs. relative age error (i.e. (output-input)/input) for the test sample of synthetic data. The blue, orange, and red points represent turn-off, main-sequence, and red giant stars, respectively. The dashed black line represents the 1:1 relation, where a star with a perfectly well estimated age should lie. The dotted lines show deviations from perfect estimation of ±50 per cent. Input parameters are randomised according to the Q25 uncertainties (see Table 1) unless specified in the top left corner. The first row plots consider a projection of only T_eff, log g, and [M/H] (labelled spec), whereas rows two to four also consider the K_s magnitude (speck); the J, H, K_s magnitudes (specjhk); and the J, H, K_s, G magnitudes (specjhkg), respectively. Finally, solid points are the stars for which the estimated relative age uncertainty is smaller than 50 per cent, whereas the semi-transparent points have estimated relative age uncertainties larger than 50 per cent. See Sect. 2.4 for further details.