  Eclipsing binary stars provide highly accurate measurements of the
fundamental physical properties of stars. They therefore serve as stringent
tests of the predictions of evolutionary models upon which most stellar age
determinations are based. Models generally perform very well in predicting
coeval ages for eclipsing binaries with main-sequence components more massive
than ~1.2 Msun; relative ages are good to ~5% or better in this mass regime.
Low-mass main-sequence stars (M < 0.8 Msun) reveal large discrepancies in the
model predicted ages, primarily due to magnetic activity in the observed stars
that appears to inhibit convection and likely causes the radii to be 10-20%
larger than predicted. In mass-radius diagrams these stars thus appear 50-90%
older or younger than they really are. Aside from these activity-related
effects, low-mass pre--main-sequence stars at ages ~1 Myr can also show
non-coevality of ~30% due to star formation effects, however these effects are
largely erased after ~10 Myr.
