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 \sim 3 Gyr .
About 15 \% of the intermediate metallicity clusters are coeval with the oldest clusters .
All the clusters with { [ Fe / H ] } > -0.8 are \sim 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 .