Optimal age sampling. Top: Relative age sensitivity of FASTAR SSP models as a function of age. The normalized age derivative of the FASTAR models becomes smaller for older ages, reflecting the fact that age variations have a progressively weaker effect on the integrated spectrum of a galaxy with increasing age. The peak contribution of the AGB (and RGB) phases appears as a sudden change in the derivative at an age of ∼1.5 Gyr. Bottom: Age sampling of the MILES models is shown in red, following their BaSTI isochrone grid. In blue, the optimized FASTAR sampling enforces a constant flux variation across all age bins.