Main: SALT2.4 light-curve stretch (x₁) as a function of the LsSFR for SNfactory SNe. The color corresponds to the probability, p_y, for the SNe Ia to be young, i.e., to have logLsSFR ≥ −10.82 (see Rigault et al. 2020). Right: p_y-weighted histogram of the SN stretches, as well as the adjusted base model; contributions of the younger and older population are shown in purple and yellow, respectively.