Top panel: Rossby number–age relations for 1.2, 1.0, 0.8, and 0.6 solar mass stars. The solid and dashed curves represent the C sequence age and the I sequence age (the first term and the second term of Eq. (5)) respectively. The same color indicates the same stellar mass and the fraction of the sample number is also presented. The vertical brown lines are Ro_sat = 0.022 and Ro_g−to−i = 0.15 respectively, separating the C, g and I region. The dotted vertical line is the separating line given by the dichotomy. The shaded region indicates the variation range of the critical Rossby number of the C-to-g and g-to-I transition (see Fig. 11), which depends on stellar mass. Bottom panel: Lower-left region in the top panel.