Rotation and activity are key parameters in stellar evolution and can be used to probe basic stellar physics . Here we present a study of rotation ( measured as projected rotational velocity v \sin i ) and chromospheric activity ( measured as H \alpha equivalent width ) based on an extensive set of high-resolution optical spectra obtained with the MIKE instrument on the 6.5 m Magellan Clay telescope . Our targets are 74 F–M dwarfs in the young stellar associations \eta Chamaeleontis , TW Hydrae , \beta Pictoris , and Tucana-Horologium , spanning ages from 6 to 30 Myr . While the H \alpha equivalent widths for most F and G stars are consistent with pure photospheric absorption , most K and M stars show measurable chromospheric emission . By comparing H \alpha equivalent width in our sample to results in the literature , we see a clear evolutionary sequence : Chromospheric activity declines steadily from the T Tauri phase to the main sequence . Using activity as an age indicator , we find a plausible age range for the Tuc-Hor association of 10–40 Myr . Between 5 and 30 Myr , we do not see evidence for rotational braking in the total sample , thus angular momentum is conserved , in contrast to younger stars . This difference indicates a change in the rotational regulation at \sim 5–10 Myr , possibly because disk braking can not operate longer than typical disk lifetimes , allowing the objects to spin up . On timescales of \sim 100 Myr there is some evidence for weak rotational braking , possibly due to stellar winds . The rotation-activity relation is flat in our sample ; in contrast to main-sequence stars , there is no linear correlation for slow rotators . We argue that this is because young stars generate their magnetic fields in a fundamentally different way from main-sequence stars , and not just the result of a saturated solar-type dynamo . By comparing our rotational velocities with published rotation periods for a subset of stars , we determine ages of 13 ^ { +7 } _ { -6 } Myr and 9 ^ { +8 } _ { -2 } Myr for the \eta Cha and TWA associations , respectively , consistent with previous estimates . Thus we conclude that stellar radii from evolutionary models by Baraffe et al . ( 2 ) are in agreement with the observed radii within \pm 15 % .