Young Galactic Globular Clusters
  Nine presently known "young" Galactic globular clusters, which have ages / 3 Gyr smaller than the oldest clusters of similar metallicity, all have RGC > 15 kpc. Furthermore these young globulars are found to have below-average luminosities. These results suggest that (1) globular cluster formation extended over a considerable period of time in the outermost part of the Galactic halo, and (2) that typical luminosities (masses) of such outer halo clusters decreased with time. 
Sidney van den Bergh