  The observed properties of young star clusters, such as the core radius and
luminosity profile, change rapidly during the early evolution of the clusters.
Here we present observations of 6 young clusters in M51 where we derive their
sizes using HST imaging and ages using deep Gemini-North spectroscopy. We find
evidence for a rapid expansion of the cluster cores during the first 20 Myr of
their evolution. We confirm this trend by including data from the literature of
both Galactic and extra-galactic embedded and young clusters, and possible
mechanisms (rapid gas removal, stellar evolutionary mass-loss, and internal
dynamical heating) are discussed. We explore the implications of this result,
focussing on the fact that clusters were more concentrated in the past,
implying that their stellar densities were much higher and relaxation times
correspondingly shorter. Thus, when estimating if a particular cluster is
dynamically relaxed, (i.e. when determining if a cluster's mass segregation is
due to primordial or dynamical processes), the current relaxation time is only
an upper-limit, with the relaxation time likely being significantly shorter in
the past.
