  Multiple star systems are commonly assumed to form coevally; they thus
provide the anchor for most calibrations of stellar evolutionary models. In
this paper we study the binary population of the Taurus-Auriga association,
using the component positions in an HR diagram in order to quantify the
frequency and degree of coevality in young binary systems. After identifying
and rejecting the systems that are known to be affected by systematic errors
(due to further multiplicity or obscuration by circumstellar material), we find
that the relative binary ages, |Delta log(tau)|, have an overall dispersion of
sigma~0.40 dex. Random pairs of Taurus members are coeval only to within
sigma~0.58 dex, indicating that Taurus binaries are indeed more coeval than the
association as a whole. However, the distribution of |Delta log(tau)| suggests
two populations, with ~2/3 of the sample appearing coeval to within the errors
(sigma~0.16 dex) and the other ~1/3 distributed in an extended tail reaching
|Delta log(tau)|~0.4-0.9 dex. To explain the finding of a multi-peaked
distribution, we suggest that the tail of the differential age distribution
includes unrecognized hierarchical multiples, stars seen in scattered light, or
stars with disk contamination; additional followup is required to rule out or
correct for these explanations. The relative coevality of binary systems does
not depend significantly on the system mass, mass ratio, or separation. Indeed,
any pair of Taurus members wider than ~10' (~0.7 pc) shows the full age spread
of the association.
