We determine ages of 71 old Open Clusters by a two-step method : we use main-squence fitting to 10 selected clusters , in order to obtain their distances , and derive their ages from comparison with our own isochrones used before for Globular Clusters .
We then calibrate the morphological age indicator \delta ( V ) , which can be obtained for all remaining clusters , in terms of age and metallicity .
Particular care is taken to ensure consistency in the whole procedure .
The resulting Open Cluster ages connect well to our previous Globular Cluster results .
From the Open Cluster sample , as well as from the combined sample , questions regarding the formation process of Galactic components are addressed .
The age of the oldest open clusters ( NGC 6791 and Be 17 ) is of the order of 10 Gyr .
We determine a delay by 2.0 \pm 1.5 Gyr between the start of the halo and thin disk formation , whereas thin and thick disk started to form approximately at the same time .
We do not find any significant age–metallicity relationship for the open cluster sample .
The cumulative age distribution of the whole open cluster sample shows a moderately significant ( \sim 2 \sigma level ) departure from the predictions for an exponentially declining dissolution rate with timescale of 2.5 Gyr .
The cumulative age distribution does not show any trend with galactocentric distance , but the clusters with larger height to the Galactic plane have an excess of objects between 2–4 and 6 Gyr with respect to their counterpart closer to the plane of the Galaxy .