Absolute ages of young stars are important for many issues in pre-main
sequence stellar and circumstellar evolution but are long recognized as
difficult to derive and calibrate. In this paper, we use literature spectral
types and photometry to construct empirical isochrones in HR diagrams for
low-mass stars and brown dwarfs in the eta Cha, epsilon Cha, and TW Hya
Associations and the beta Pic and Tuc-Hor Moving Groups. A successful theory of
pre-main sequence evolution should match the shapes of the stellar loci for
these groups of young stars. However, when comparing the combined empirical
isochrones to isochrones predicted from evolutionary models, discrepancies lead
to a spectral type (mass) dependence in stellar age estimates. Improved
prescriptions for convection and boundary conditions in the latest models of
pre-main sequence models lead to a significantly improved correspondence
between empirical and model isochrones, with small offsets at low temperatures
that may be explained by observational uncertainties or by model limitations.
Independent of model predictions, linear fits to combined stellar loci of these
regions provide a simple empirical method to order clusters by luminosity with
a reduced dependence on spectral type. Age estimates calculated from various
sets of modern models that reproduce Li depletion boundary ages of the beta Pic
Moving Group also imply a ~4 Myr age for the low mass members of the Upper Sco
OB Association, which is younger than the 11 Myr age that has been recently
estimated for intermediate mass members.Comment: Accepted by ApJ, 18 page
