Abstract
We have studied the H α (equivalent widths and fluxes) of late-type members of three young open clusters and used these data to define a empirical criterion which allows, from the statistical point of view, to distinguish between Classical and Weak-line Tstars. This criterion is based on the saturation of the chromospherical activity and can be used in combination with low resolution spectroscopy. We have extend this criterion into the substellar domain and apply it several star forming regions of different ages. Our comparison shows that the fraction of accretor decline very fast with age both in the stellar and the substellar domain, becoming negligible at about 10.
