  We present accurate relative ages for a sample of 55 Galactic globular
clusters. The ages have been obtained by measuring the difference between the
horizontal branch and the turnoff in two, internally photometrically
homogeneous databases. The mutual consistency of the two data sets has been
assessed by comparing the ages of 16 globular clusters in common between the
two databases. We have also investigated the consistency of our relative age
determination within the recent stellar model framework. All clusters with
[Fe/H]<-1.7 are found to be old, and coeval, with the possible exception of two
objects, which are marginally younger. The age dispersion for the metal poor
clusters is 0.6 Gyr (rms), consistent with a null age dispersion. Intermediate
metallicity clusters (-1.7<[Fe/H]<-0.8) are on average 1.5 Gyr younger than the
metal poor ones, with an age dispersion of 1.0 Gyr (rms), and a total age range
of ~3 Gyr. About 15% of the intermediate metallicity clusters are coeval with
the oldest clusters. All the clusters with [Fe/H]>-0.8 are ~1 Gyr younger than
the most metal poor ones, with a relatively small age dispersion, though the
metal rich sample is still too small to allow firmer conclusions. There is no
correlation of the cluster age with the Galactocentric distance. We briefly
discuss the implication of these observational results for the formation
history of the Galaxy.
