Upper panel: comparison of the Ro–R_X distribution shown in Fig. 4 with the stars in the ∼12 Myr cluster h Per with the black circles and green triangles showing X-ray detections and upper limits from Argiroffi et al. (2016). We recalculate the Rossby numbers using the convective turnover times given by Spada et al. (2013) assuming an age of 12 Myr. Lower panel: comparison of the Ro–R_X distribution shown in Fig. 4 with a sample of fully convective M dwarfs from Jeffries et al. (2011) and Wright et al. (2018), with detections and upper limits shown as circles and triangles.