Age–metallicity relation of the initial stellar mass distribution derived from the orbit superposition modelling of APOGEE data. The bottom panel displays the age-metallicity relation after applying a correction to stellar ages of the young α–rich (YAR) and metal-poor populations. These revised ages were resampled from the high-α population, as described in Section 3.2. The corrected YAR and metal-poor stars together account for approximately 13% of the total stellar mass of the Galaxy. In both panels, the white histograms trace the age mass density distribution, while the colour bar illustrates the normalised, mass-weighted density of the age–metallicity relation. Fig. A.1 provides an additional comparison of the original and adjusted ages.