An example of isochrones fit (see text) to determine age differences among the three sub-populations studied in this paper. Left panel: the literature values of E(B − V) and (m − M)_V for ω Cen are used to derive age differences; the oldest populations turns out to be 16 Gr old. Centre panel: a cosmological age of 13 Gyr is forced for the oldest populations, altering E(B − V) and (m − M)_V but with no significant effect on the resulting age differences; the shape of the isochrones is now in worse agreement with the data. Right panel: NGC 6397 is shown for comparison. The letters A, B, and C, which appear in the two leftmost panels, indicate the approximate location of the SGB branches as defined by Villanova et al. (2007).