  We present time-series radial-velocity and photometric observations of a
solar-type double-lined eclipsing binary star (V12) near the turnoff in the old
open cluster NGC188. We use these data to determine the spectroscopic orbit and
the photometric elements for V12. From our analysis we determine accurate
masses (Mp=1.103+-0.007Msun, Ms=1.081+-0.007Msun) and radii (R
=1.424+-0.019Rsun, Rs=1.373+-0.019Rsun) for the primary (p) and secondary (s)
binary components. We adopt a reddening of E_(B-V)=0.087 for NGC188, and derive
component effective temperatures of 5900+-100K and 5875+-100K, respectively,
for the primary and secondary stars. From their absolute dimensions, the two
components of V12 yield identical distance moduli of Vo-MV=11.24+-0.09,
corresponding to 1770+-75pc. We determine an age of 6.2+-0.2Gyr for V12 and
NGC188, from a comparison with theoretical isochrones in the mass-radius
diagram. This age is independent of distance, reddening, and color-temperature
transformations. We use isochrones from Victoria-Regina (VRSS) and Yonsei-Yale
(Y2) with [Fe/H]=-0.1 and [Fe/H]=0.0. From the solar metallicity isochrones, an
age of 6.4Gyr provides the best fit to the binary components for both sets of
models. For the isochrones with [Fe/H]=-0.1, ages of 6.0Gyr and 5.9Gyr provide
the best fits for the VRSS and Y2 models, respectively. We use the distance and
age estimates for V12, together with best estimates for the metallicity and
reddening of NGC188, to investigate the locations of the corresponding VRSS and
Y2 isochrones relative to cluster members in the color-magnitude diagram.
Plausible changes in model metallicity and distance to better match the
isochrones to the cluster sequences, result in a range of ages for NGC188 that
is more than 3 times that resulting from our analysis of V12.
