Simulated L_X/L_bol ratio distributions of solar mass stars in open clusters with ages of 5,13, 85, 112, 240, 625, 1000, 2400, and 4000 Myr. The results are compared with measured L_X/L_bol histograms of 0.9–1.1 M_⊙ stars in NGC 2362, h Per, α Per, the Pleiades, Praesaepe and the Hyades scaled to the 979 stars used in the simulation. The short-term variability of the magnetic activity is described by a normal distribution of the short-term X-ray emission with a variance of ⟨ L_X⟩/6 around the average X-ray luminosity ⟨ L_X⟩ derived from aparametric model of rotation evolution for solar mass stars.