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[Abridged] We have studied the properties of the stellar populations in thefield of the NGC346 cluster in the Small Magellanic Cloud, using a novelself-consistent method that allows us to reliably identify pre-main sequence(PMS) objects actively undergoing mass accretion, regardless of their age. Themethod does not require spectroscopy and combines broad-band V and I photometrywith narrow-band Halpha imaging to identify all stars with excess Halphaemission and derive the accretion luminosity Lacc and mass accretion rate Maccfor all of them. The application of this method to existing HST/ACS photometryof the NGC346 field has allowed us to identify and study 680 bona-fide PMSstars with masses from ~0.4 to ~4 Msolar and ages in the range from ~1 to ~30Myr. This is the first study to reveal that, besides a young population of PMSstars (~ 1 Myr old), in this field there is also an older population of PMSobjects with a median age of ~20 Myr. We provide for all of them accuratephysical parameters. We study the evolution of the mass accretion rate as afunction of stellar parameters and find that logMacc ~ -0.6 Log t + Log m + c,where t is the age of the star, m its mass and c a quantity that is higher atlower metallicity. The high mass accretion rates that we find suggest that aconsiderable fraction of the stellar mass is accreted during the PMS phase andthat PMS evolutionary models that do not account for this effect willsystematically underestimate the true age when compared with the observations.

Photometric determination of the mass accretion rates of pre-main sequence stars. II. NGC346 in the Small Magellanic Cloud