Top row, left: 2D density of probability in the age vs. α_ml plane from samples of 35 RGB stars (true age of 7.5 Gyr). Parameters are recovered by means of the pure geometrical method (see text). The cross marks the position of the true values, while the filled circle marks the median of the recovered ones. Middle: same as in the left panel, but in the age vs. ΔY/ΔZ plane. Right: as in the left panel, but in the age vs. Z plane. Middle row: as in the top row, but for maximum likelihood recovery. Bottom row: as in the top row, but for SCEPtER individual recovery. For ease of comparison, the corresponding panels in the three subsequent figures (Figs. 2–4) share the same colour key.