  Rotation periods and projected equatorial velocities of pre-main-sequence
(PMS) stars in star forming regions can be combined to give projected stellar
radii. Assuming random axial orientation, a Monte-Carlo model is used to
illustrate that distributions of projected stellar radii are very sensitive to
ages and age dispersions between 1 and 10 Myr which, unlike age estimates from
conventional Hertzsprung-Russell diagrams, are relatively immune to
uncertainties due to extinction, variability, distance etc. Application of the
technique to the Orion Nebula cluster reveals radius spreads of a factor of
2--3 (FWHM) at a given effective temperature. Modelling this dispersion as an
age spread suggests that PMS stars in the ONC have an age range larger than the
mean cluster age, that could be reasonably described by the age distribution
deduced from the Hertzsprung-Russell diagram. These radius/age spreads are
certainly large enough to invalidate the assumption of coevality when
considering the evolution of PMS properties (rotation, disks etc.) from one
young cluster to another.
