Left column: envelopes of age estimate relative errors due to sampling from synthetic grids with different values of ΔY/ΔZ. Ages are estimated on the standard grid with ΔY/ΔZ = 2. Middle column: same as the left column, but for sampling from synthetic grids with different values of the mixing-length parameter α_ml. Ages are estimated on the standard grid with α_ml = 1.74. Right column: same as the left column, but for sampling from synthetic grids with different values of radiative opacity k_r. Ages are estimated on the standard grid.