  We use the framework developed as part of the MESA Isochrones and Stellar
Tracks (MIST) project to assess the utility of several types of observables in
jointly measuring the age and 1D stellar model parameters in star clusters. We
begin with a pedagogical overview summarizing the effects of stellar model
parameters, such as the helium abundance, mass-loss efficiency, and the mixing
length parameter, on observational diagnostics including the color-magnitude
diagram, mass-radius relation, and surface abundances, amongst others. We find
that these parameters and the stellar age influence observables in
qualitatively distinctive, degeneracy-breaking ways. To assess the current
state of affairs, we use the recent Gaia Data Release 2 (DR2) along with data
from the literature to investigate three well-studied old open
clusters---NGC6819, M67, NGC6791---as case studies. Although there is no
obvious tension between the existing observations and the MIST models for
NGC6819, there are interesting discrepancies in the cases of M67 and NGC6791.
At this time, parallax zero point uncertainties in Gaia DR2 remain one of the
limiting factors in the analysis of these clusters. With a combination of
exquisite photometry, parallax distances, and cluster memberships from Gaia at
the end of its mission, we anticipate precise and accurate ages for these and
other star clusters in the Galaxy.
