I propose and apply a method for deriving ages of the metal-poor globular clusters ( GCs ) in a distance-independent way , which is based on age indicator related to the main sequence in the two–color diagrams with U - B index .
Age of the metal-poor GC NGC 6397 has been estimated , using Yale isochrones of Demarque et al .
( 1996 ) , to be close to 16 Gyr provided the cluster metallicity is near [ Fe / H ] = -2.0 .