  In the era of large stellar spectroscopic surveys, there is emphasis on
deriving not only stellar abundances but also ages for millions of stars. In
the context of Galactic archeology, stellar ages provide a direct probe of the
formation history of the Galaxy. We use the stellar evolution code MESA to
compute models with atomic diffusion--with and without radiative
acceleration--and extra mixing in the surface layers. The extra mixing consists
of both density-dependent turbulent mixing and envelope overshoot mixing. Based
on these models we argue that it is important to distinguish between initial,
bulk abundances (parameters) and current, surface abundances (variables) in the
analysis of individual stellar ages. In stars that maintain radiative regions
on evolutionary timescales, atomic diffusion modifies the surface abundances.
We show that when initial, bulk metallicity is equated with current, surface
metallicity in isochrone age analysis the resulting stellar ages can be
systematically over-estimated by up to 20%. The change of surface abundances
with evolutionary phase also complicates chemical tagging, the concept that
dispersed star clusters can be identified through unique, high-dimensional
chemical signatures. Stars from the same cluster, but in different evolutionary
phases, will show different surface abundances. We speculate that calibration
of stellar models may allow us to estimate not only stellar ages but also
initial abundances for individual stars. In the meantime, analyzing the
chemical properties of stars in similar evolutionary phases is essential to
minimize the effects of atomic diffusion in the context of chemical tagging.
