Colour-period diagrams for M 34 (Meibom et al. 2011), NGC 3532 (this work), and M 48 (Barnes et al. 2015; from left to right) showing the period evolution in this 220 − 450 Myr age range. Fiducial lines for NGC 3532 (dashed) are reproduced in the panels for M 34 and M 48. We see that the M 34 rotation period distribution is mostly below the NGC 3532 fiducials while the M 48 distribution is above them. This informs us that M 34 is younger than NGC 3532 while M 48 is older.