  Main sequence turnoff magnitudes from the recent set of Yale isochrones
(Chaboyer \ea 1995) have been combined with a variety of relations for the
absolute magnitude of RR Lyr stars (\mvrr) to calibrate age as a function of
the difference in magnitude between the main sequence turn-off and the
horizontal branch (\dv). A best estimate for the calibration of \mvrr is
derived from a survey of the current literature: $\mvrr = 0.20 \feh + 0.98$.
This estimate, together with other calibrations (with slopes ranging from 0.15
to 0.30) has been used to derive \dv ages for 43 Galactic globular clusters.
Independent of the choice of \mvrr, there is no strong evidence for an
age-Galacto\-centric distance relationship among the 43 globular clusters.
However, an age-metallicity relation exists, with the metal-poor clusters being
the oldest. A study of the age distribution reveals that an age range of 5 Gyr
exists among the bulk of the globular clusters. In addition, about 10\% of the
sample are substantially younger, and including them in the analysis increases
the age range to 9 Gyr. Once again, these statements are independent of the
\mvrr ~relation. Evidence for age being the second parameter governing
horizontal branch morphology is found by comparing the average \dv ~age of the
second parameter clusters to the normal clusters. The second parameter clusters
are found to be on average 2 - 3 Gyr younger than the other clusters, which is
consistent with age being the second parameter. These results suggest that
globular clusters were formed over an extended period of time, with
progressively more metal-rich globular clusters ($\feh \ga -1.7$) being formed
at later times.
