We present revised stellar ages for 23 pre-main-sequence (PMS) K- and M-type stars in the Upper Scorpius (Upper Sco) star-forming region, derived using stellar dynamical masses to constrain isochronal ages from five PMS stellar evolutionary models. We find that mass-constrained stellar ages for all model sets are more consistent with the older, ∼8–11 Myr age for Upper Sco derived using earlier-type stars. Additionally, applying the independent mass constraint to isochronal ages tends to (1) increase stellar ages for most model sets, and (2) decrease age scatter for individual sources between model sets. Models that account for global magnetic fields consistently provide the best match to our observations: they change comparatively little when the mass constraint is applied, and produce 9–10 Myr ages under both unconstrained and mass-constrained conditions. Most standard (nonmagnetic) models produce younger ages (3–5 Myr) when unconstrained, but older ages (6–9 Myr) when constrained by dynamical mass. Our results are consistent with recent literature findings that suggest median disk lifetimes may be ≳2× longer than previously thought.