  Ages have been derived for 55 globular clusters (GCs) from overlays of
isochrones onto the turnoff photometry, assuming distances based on fits of
zero-age horizontal branch (ZAHB) models to the lower bound of the observed
distributions of HB stars. The error bar arising just from the "fitting" of
ZAHBs and isochrones is ~ +/- 0.25 Gyr, while that associated with distance and
chemical abundance uncertainties is ~ +/- 1.5-2 Gyr. Ages vary from mean values
of ~12.5 Gyr at [Fe/H] < -1.7 to ~11 Gyr at [Fe/H] > -1.0. At intermediate
metallicities, the age-metallicity relation (AMR) appears to be bifurcated: one
branch apparently contains clusters with disk-like kinematics, whereas the
other branch is populated by clusters with halo-type orbits. There is no
apparent dependence of age on Galactocentric distance (R_G) nor is there a
clear correlation of HB type with age. Subtle variations in the subgiant branch
(SGB) slopes of [Fe/H] < -1.5 GCs are tentatively attributed to helium
abundance differences. Curiously, GCs with steep "M13-like" SGBs tend to be
massive systems, located at small R_G, that show the strongest evidence for
multiple stellar populations. The others are typically low-mass systems that,
at the present time, should not be able to retain the matter lost by
mass-losing stars. The apparent separation of the two groups in terms of their
present-day gas retention properties is difficult to understand if all GCs were
initial ~20 times their current masses. The lowest mass systems may have never
been able to retain enough gas to produce a significant population of
second-generation stars; in this case, the observed light element abundance
variations were presumably present in the gas out of which the observed cluster
stars formed.
