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We investigate the evolution of X-ray stellar activity from the age of theyoungest known star forming regions (SFR), < 1Myr, to about 100 Myr, i.e. thezero age main sequence (ZAMS) for a ~ 1M_sun star. We consider five SFR ofvarying age (Rho Ophiuchi, the Orion Nebula Cluster, NGC 2264, Chamaeleon I,and Eta Chamaeleontis) and two young clusters (the Pleiades and NGC 2516).Optical and X-ray data for these regions are retrieved both from archivalobservations and recent literature, and reanalyzed here in a consistent mannerso to minimize systematic differences in the results. We study trends of L_Xand L_X/L_bol as a function of stellar mass and association age. For low massstars (M < 1M_sun) we observe an increase in L_X/L_bol in the first 3-4 Myr anda subsequent leveling off at the saturation level (L_X/L_bol ~ -3). Slowlyevolving very low mass stars then retain saturated levels down to the oldestages here considered, while for higher mass stars activity begins to decline atsome age after ~10^7 years. We find our data consistent with the followingtentative picture: low mass PMS stars with no circumstellar accretion disk havesaturated activity, consistently with the activity-Rossby number relationderived for MS stars. Accretion and/or the presence of disks somehow lowers theobserved activity levels; disk dissipation and/or the decrease of massaccretion rate in the first few Myrs of PMS evolution is therefore responsiblefor the observed increase of L_X/L_bol with time.Comment: 17 pages, 8 figures, accepted for publication in A&

Time evolution of X-ray coronal activity in PMS stars; a possible relation with the evolution of accretion disks