  Young stars form in associations, meaning that young stellar associations
provide an ideal environment to measure the age of a nominally coeval
population. Isochrone fitting, which is the typical method for measuring the
age of a coeval population, can be impacted by observational biases that
obscure the physical properties of a population. One feature in isochrone fits
of star-forming regions is an apparent mass-dependent age gradient, where
lower-mass stars appear systematically younger than higher-mass stars.
Starspots and stellar multiplicity are proposed mechanisms for producing the
mass-dependent age gradient, but the relative importance of starspots versus
multiplicity remains unclear. We performed a synthetic red-optical
low-resolution spectroscopic survey of a simulated analog to a 10 Myr stellar
association including mass-dependent multiplicity statistics and age-dependent
starspot coverage fractions. We found that undetected starspots alone do not
produce an apparent mass-dependent age gradient, but instead uniformly reduce
the average measured age of the population. We also found that binaries
continue to produce an apparent mass-dependent age gradient, and introduce more
scatter in the age measurement than spots, but are easily removed from the
population as long as there are good distance measurements to each target. We
conclude that it is crucial to incorporate treatments of both starspots and
undetected stellar multiplicity into isochrone fits of young stellar
associations to attain reliable ages.
