Formation redshift functions (FzF). Top left: formation redshift functions computed from the galaxies in 3 observed redshift bins: 2 <z_obs ≤ 3 (black crosses), 3 <z_obs ≤ 4 (blue points), and z_obs ≥ 4.0 (red triangles). The three other panels correspond to the comparison of the FzFs corresponding to the 3 different age definitions used in this paper in increasing redshift bins. From top right to bottom right we compare the FzFs in the following observed redshift bins: 2 <z ≤ 3, 3 <z ≤ 4 and z ≥ 4.0.