Determination of NGC 1835 absolute age. Left panels: trend of the χ² values obtained from the isochrone fitting of the MS-TO region of the models (top and bottom panels, respectively) computed for [Fe/H] = −1.7, [α/Fe]= + 0.4, Y = 0.248, and ages ranging between 10 Gyr and 15 Gyr. Right panels: decontaminated CMD of NGC 1835 with the BaSTI and PARSEC isochrones superposed (top and bottom panels, respectively), corresponding to the best-fit age (12.5 Gyr, solid lines) and the estimated age interval (±1 Gyr, dashed lines).