  A model of protostar mass and luminosity evolution in clusters gives new
estimates of cluster age, protostar birthrate, accretion rate and mean
accretion time. The model assumes constant protostar birthrate, core-clump
accretion, and equally likely accretion stopping. Its parameters are set to
reproduce the initial mass function, and to match protostar luminosity
distributions in nearby star-forming regions. It obtains cluster ages and
birthrates from the observed numbers of protostars and pre-main sequence (PMS)
stars, and from the modal value of the protostar luminosity. In 31 embedded
clusters and complexes the global cluster age is 1-3 Myr, matching available
estimates based on optical spectroscopy and evolutionary tracks. This method of
age estimation is simpler than optical spectroscopy, and is more useful for
young embedded clusters where optical spectrocopy is not possible. In the
youngest clusters, the protostar fraction decreases outward from the densest
gas, indicating that the local star-forming age increases outward from a few
0.1 Myr in small protostar-dominated zones to a few Myr in large PMS-dominated
zones.
