“Best sample” of X-ray sources based on estimates of masses and ages for 340 2MASS counterparts (see Table B.2). This sample, which takes into account the incompleteness of our sample below ~0.5 M_⊙, are present in the grey area. The empty area between dotted lines corresponding to the mass range 0.5–0.7 M_⊙ and age >10 Myr is interpreted as the decline of magnetic activity of low-mass young stars with age (see Sect. 7.2).