We investigate the evolution of X-ray stellar activity from the age of the youngest known star forming regions ( SFR ) , \lesssim 1 Myr , to about 100 Myr , i.e .
the zero age main sequence ( ZAMS ) for a \sim 1 M _ { \odot } star .
We consider five SFR of varying 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 archival observations and recent literature , and reanalyzed here in a consistent manner so to minimize systematic differences in the results .

We study trends of L _ { X } and L _ { X } / L _ { bol } as a function of stellar mass and association age .
For low mass stars ( M \lesssim 1 M _ { \odot } ) we observe an increase in L _ { X } / L _ { bol } in the first 3-4 Myr and a subsequent leveling off at the saturation level ( L _ { X } / L _ { bol } \sim - 3 ) .
Slowly evolving very low mass stars then retain saturated levels down to the oldest ages here considered , while for higher mass stars activity begins to decline at some age after \sim 10 ^ { 7 } years .

We find our data consistent with the following tentative picture : low mass PMS stars with no circumstellar accretion disk have saturated activity , consistently with the activity-Rossby number relation derived for MS stars .
Accretion and/or the presence of disks somehow lowers the observed activity levels ; disk dissipation and/or the decrease of mass accretion rate in the first few Myrs of PMS evolution is therefore responsible for the observed increase of L _ { X } / L _ { bol } with time .