Average metallicity of the Milky Way galaxy as a function of age. The light-weighted average magnesium abundance ([Mg/H]_LW) of mono-age populations, which is integrated over the entire Galaxy and comparable to the gas-phase metallicity measurements of external galaxies at different redshifts, are shown in blue circles. [Mg/H]_LW of the Galaxy within and beyond 4 kpc are shown in gray stars and diamonds, respectively. Black solid and green dashed–dotted lines depict the prediction of chemical evolution model involving a recent gas-accretion event (J. Lian et al. 2020b) and data-driven model best-fitting the resolved age–[Fe/H] relations across the Galaxy (H. Zhang et al. 2026), respectively. The red dashed curve shows the late-accretion model with no radial migration. The mass-weighted integrated iron abundance ([Fe/H]_MW) at each lookback time, which is comparable to the stellar metallicity of galaxies, are shown in orange squares. Error bars denote 1σ uncertainty of the measurements.