Age dependence of the radial metallicity gradient in the Galactic disc with LAMOST and Gaia

We investigate the stellar radial metallicity ([Fe/H]) gradient–age relation (MGAR) of the Galactic disc using a solar-neighbourhood sample of 580 037 FGK dwarf and turn-off stars from LAMOST DR7, Gaia DR3, and the corrected StarHorse age catalogue. The fiducial MGAR is non-monotonic, with relatively strong negative gradients at intermediate ages followed by substantial flattening towards older populations. The most negative gradient occurs at 6–8 Gyr in the adopted 2-Gyr bins, although the 4–6 Gyr slope is statistically similar, indicating a broad intermediate-age turnover rather than a sharply defined transition. The flattening persists within the low-α population, indicating that chemical-population mixing alone cannot explain the full-sample relation, and is also recovered under alternative guiding-radius definitions, radial fitting ranges, vertical-orbit selections, propagation of the reported random age uncertainties, and an alternative stellar-age scale. In contrast, controlling the age-dependent Teff–log g distribution substantially reduces the absolute gradient amplitudes and modifies the younger-age behaviour. A same-star comparison with APOGEE DR17 further shows that the broad age dependence of the controlled MGAR is recovered using an external [Fe/H] scale. We therefore identify the intermediate-to-old-age flattening as the most robust feature of the observed MGAR, whereas the precise location and depth of the intermediate-age minimum and the detailed younger-age morphology are less stable. These results indicate that stellar-parameter selection can contribute to the diversity of MGAR morphologies reported in previous studies.