Based on a new large , homogeneous photometric database of 34 Galactic globular clusters ( + Pal 12 ) , a set of distance and reddening independent relative age indicators has been measured .
The observed \delta ( V - I ) _ { @ 2.5 } and \Delta V ^ { HB } _ { TO } vs. metallicity relations have been compared to the relations predicted by two recent updated libraries of isochrones .
Using these models and two independent methods , we have found that self-consistent relative ages can be estimated for our GGC sample .
In turn , this demonstrates that the models are internally self-consistent .

Based on the relative age vs. metallicity distribution , we conclude that : ( a ) there is no evidence of an age spread for clusters with [ Fe/H ] < -1.2 , all the clusters of our sample in this range being old and coeval ; ( b ) for the intermediate metallicity group ( -1.2 \leq [ Fe/H ] < -0.9 ) there is a clear evidence of age dispersion , with clusters up to \sim 25 \% younger than the older members ; and ( c ) the clusters within the metal rich group ( [ Fe/H ] \geq - 0.9 ) seem to be coeval within the uncertainties ( except Pal 12 ) , but younger ( \sim 17 \% ) than the bulk of the Galactic globulars .
The latter result is totally model dependent .

From the Galactocentric distribution of the GGC ages , we can divide the GGCs in two groups : The old coeval clusters , and the young clusters .
The second group can be divided into two subgroups , the “ real young clusters ” and the “ young , but model dependent ” , which are within the intermediate and high metallicity groups , respectively .
From this distribution , we can present a possible scenario for the Milky Way formation : The GC formation process started at the same zero age throughout the halo , at least out to \sim 20 kpc from the Galactic center .
According to the present stellar evolution models , the metal-rich globulars are formed at a later time ( \sim 17 \% lower age ) .
And finally , significantly younger halo GGCs are found at any R _ { GC } > 8 kpc .
For these , a possible scenario associated with mergers of dwarf galaxies to the Milky Way is suggested .

\keywords Hertzsprung-Russell ( HR ) Diagram – Stars : Population II – Globular Clusters : General – The Galaxy : Evolution – The Galaxy : Formation .