Comparisons between observed and predicted distributions of L_X at ages of 150 Myr (left) and 620 Myr (right). The grey symbols show predicted distributions calculated using Eq. (1) and the distributions of rotation rates derived by Johnstone et al. (2015). The red circles and green triangles show detections and upper limits for stars in the Pleiades (left) and Hyades (right). The horizontal blue lines show detection thresholds. The upper line of stars in the theoretical distributions is caused by the stars whose rotation rates lie above the mass dependent saturation threshold.