  Using up-to-date stellar models and isochrones we determine the age of 25
galactic halo clusters. The clusters are distributed into four groups according
to metallicity. We measure the absolute age of a reference cluster in each
group, and then find the relative ages of the other clusters relative to this
one. This combination yields the most reliable results. We find that the oldest
cluster group on average is 11.8+-0.9 Gyr or 12.3+-0.3 Gyr old, depending on
whether we include Arp2 and Rup106. The average age of all clusters is about
10.5 Gyr. Questions concerning a common age for all clusters and a relation
between metallicity and age are addressed. The groups of lower metallicity
appear to be coeval, but our results indicate that globally the sample has an
age spread without a simple age-metallicity relation.
