Top panel: L_acc vs. L_∗ as in Fig. 12, with colours indicating the different age of the objects as bottom panel of Fig. 13. Bottom panel: simulation of the L_acc vs. L_∗ temporal evolution for YSOs of different masses (0.4, 0.7, and 1 M_⊙), derived considering the evolutionary tracks from Siess et al. (2000) and a viscous disc evolution with Ṁ_acc ∝ t^−η, with η = 1.2. Different colours indicate different age, ranging from 10⁴ to 10⁷ yr.