  We determine the relative ages of the open cluster NGC 188 and selected
Hipparcos field stars by isochrone fitting, and compare them to the age of the
thick disk globular cluster 47 Tuc. The best fit age for NGC 188 was determined
to be $6.5 \pm 1.0$ Gyr. The solar metallicity Hipparcos field stars yielded a
slightly older thin disk age, $7.5 \pm 0.7$ Gyr. Two slightly metal-poor ($\feh
= -0.22$) field stars whose kinematic and orbital parameters indicate that they
are members of the thin disk were found to have an age of $9.7\pm 0.6$ Gyr. The
age for 47 Tuc was determined to be $12.5 \pm 1.5$ Gyr. All errors are internal
errors due to the uncertainty in the values of metallicity and reddening. Thus,
the oldest stars dated in the thin disk are found to be $2.8\pm 1.6$ Gyr
younger than 47 Tuc. Furthermore, as discussed by \citet{Chb99} 47 Tuc has a
similar age to three globular clusters located in the inner part of the
Galactic halo, implying that star formation in the thin disk started within
$2.8\pm 1.6$ Gyr of star formation in the halo.
