Radial metallicity ([M/H]/[Fe/H]) gradient variations with stellar ages of the LAMOST-MRS young stellar sample in this work and those presented by Wang et al. (2019), Xiang et al. (2015), and Vickers et al. (2021). Radial metallicity gradients estimated using young objects (OB stars, Cepheid variables, H II regions, and OCs) from previous works are also shown using different symbols as labelled in the figure. We note that the x-axis is logarithmic.