Age-metallicity relation for simulated galaxies: a) negative age gradients (upper-left galaxy: ∇_age = −0.41;∇_Z = −0.34; lower-left galaxy: ∇_age = −0.40;∇_Z = −0.14) and b) positive age gradients (upper-right galaxy: ∇_age = 0.32;∇_Z = −0.34; lower-right galaxy: ∇_age = 0.10;∇_Z = −0.14). The colours denote the distance to the centre of mass in units of the r_eff. Each point depicts a star particle representing a single SP. The relation between old and new SPs clearly differs and is responsible for the different age profiles in the simulations.