uvby(-$\beta$) photometry of high-velocity and metal-poor stars X. Stars   of very low metal abundance: observations, reddenings, metallicities,   classifications, distances, and relative ages
  uvby(--$\beta$) photometry has been obtained for an additional 411 very metal-poor stars selected from the HK survey, and used to derive basic parameters such as interstellar reddenings, metallicities, photometric classifications, distances, and relative ages... These very metal-poor stars are compared to M92 in the c_0,(b-y)_0 diagram, and evidence is seen for field stars 1-3 Gyrs younger than this globular cluster. The significance of these younger very metal-poor stars is discussed in the context of Galactic evolution, mentioning such possibilities as hierarchical star-formation/mass-infall of very metal-poor material and/or accretion events whereby this material has been acquired from other (dwarf) galaxies with different formation and chemical-enrichment histories 
W. J. Schuster, T. C. Beers, R. Michel, P. E. Nissen, and G. Garcia