Effect on the initial [Fe/H] (top) and [O/Fe] (bottom) gradients for different stellar age groups. The solid and dotted color curves show the initial and final states, respectively. Note that while strong flattening is observed for the older populations, the metallicity gradient for the youngest stars (age  < 2) is hardly affected at r ≲ 12 kpc, thus justifying the use of our chemical model, which uses this as a constraint.