  When determining absolute ages of identifiably young stellar populations,
results strongly depend on which stars are studied. Cooler (K, M) stars
typically yield ages that are systematically younger than warmer (A, F, G)
stars by a factor of two. I explore the possibility that these age
discrepancies are the result of magnetic inhibition of convection in cool young
stars by using magnetic stellar evolution isochrones to determine the age of
the Upper Scorpius subgroup of the Scorpius-Centaurus OB Association. A median
age of 10 Myr consistent across spectral types A through M is found, except for
a subset of F-type stars that appear significantly older. Agreement is shown
for ages derived from the Hertzsprung-Russell diagram and from the empirical
mass-radius relationship defined by eclipsing multiple-star systems. Surface
magnetic field strengths required to produce agreement are of order 2.5 kG and
are predicted from a priori estimates of equipartition values. A region in the
HR diagram is identified that plausibly connects stars whose structures are
weakly influenced by the presence of magnetic fields with those whose
structures are strongly influenced by magnetic fields. The models suggest this
region is characterized by stars with rapidly thinning outer convective
envelopes where the radiative core mass is greater than 75% of the total
stellar mass. Furthermore, depletion of lithium predicted from magnetic models
appears in better agreement with observed lithium equivalent widths than
predictions from non-magnetic models. These results suggest that magnetic
inhibition of convection plays an important role in the early evolution of
low-mass stars and that it may be responsible for noted age discrepancies in
young stellar populations.
