  We have compiled and studied photometric and spectroscopic data published in
the literature of several star forming regions and young open clusters (Orion,
Taurus, IC348, Sco-Cen Complex, Chamaeleon I, TW Hya association, sigma Orionis
cluster, IC2391, alpha Per cluster and the Pleiades). Our goal was to seek the
definition of a simple empirical criterion to classify stars or brown dwarfs
which are accreting matter from a disk on the sole basis of low-resolution
optical spectroscopic data. We show that using Halpha equivalent widths and
spectral types we can statistically classify very young stars and brown dwarfs
as classical T Tauri stars and substellar analogs. As a boundary between
accreting and non accreting objects, we use the saturation limit of
chromospheric activity at Log L(Halpha)/L(bol)=-3.3 (determined in the open
clusters). We discuss the uncertainties in the classification scheme due to the
occurrence of flares. We have used this spectroscopic empirical criterion to
classify objects found in the literature, and we compute the fraction of
accreting objects in several star forming regions. The fraction of accreting
objects appears to decrease from about 50% to about 5% from 1 Myr to 10 Myr for
both stars and brown dwarfs.
