  The determination of age is a critical component in the study of a population
of stellar clusters. In this letter we present a new absolute age indicator for
young massive star clusters based on J-H colour. This novel method identifies
clusters as older or younger than 5.7 +/- 0.8 Myr based on the appearance of
the first population of red supergiant stars. We test the technique on the
stellar cluster population of the nearby spiral galaxy, M83, finding good
agreement with the theoretical predictions. The localisation of this technique
to the near-IR promises that it may be used well into the future with space--
and ground--based missions optimised for near-IR observations.
