  We present homogeneous age determinations for a large sample of 55 Galactic
globular clusters, which constitute about 30% of the total Galactic population.
A study of their age distribution reveals that all clusters from the most metal
poor ones up to intermediate metallicities are coeval, whereas at higher [Fe/H]
an age spread exists, together with an age-metallicity relationship. At the
same time, all clusters within a certain galactocentric distance appear coeval,
whereas an age spread is present further away from the Galactic centre, without
any correlation with distance. The precise value of [Fe/H] and galactocentric
distance for the onset of the age spread and the slope of the age-metallicity
relationship are strongly affected by the as yet uncertain [Fe/H] scale. We
discuss how differences in the adopted [Fe/H] scale and cluster sample size may
explain discrepant results about the clusters age distribution reached by
different authors. Taking advantage of the large number of objects included in
our sample, we also tested the possibility that age is the global second
parameter which determines the Horizontal Branch morphology, and found
indications that age could explain the global behaviour of the second parameter
effect.
