  We present a new method to determine the age spread of resolved stellar
populations in a starburst cluster. The method relies on a two-step process. In
the first step, kinematic members of the cluster are identified based on
multi-epoch astrometric monitoring. In the second step, a Bayesian analysis is
carried out, comparing the observed photometric sequence of cluster members
with sets of theoretical isochrones. When applying this methodology to optical
and near-infrared high angular resolution Hubble Space Telescope (HST) and
adaptive optics observations of the ~5 Myr old starburst cluster Westerlund 1
and ~2 Myr old starburst cluster NGC 3603 YC, we derive upper limits for the
age spreads of 0.4 and 0.1 Myr, respectively. The results strongly suggest that
star formation in these starburst clusters happened almost instantaneously.
