Observed mass-luminosity ratio vs. age of the Galactic open clusters in the optical (V) and NIR (K_s) photometric passbands. The dots are data for individual clusters, the bars indicate the individual errors in the data. The open dots mark extended clusters excluded from the analysis (see text). The large black dots show the running median of the M/L ratio computed with a (log t)-bin of 0.5 and a step of 0.25. The straight line in the left panel is the empirical relation from Battinelli et al. (1994).