We exploit stellar population models of absorption line indices in the ultraviolet to assess their power in determining the age of the stellar populations in galaxies. We focus in particular on features that can differentiate between an old UV bright population, contributing to the UV upturn, and a young population due to recent star formation. We use a system of 8 indices between 2200 – 3200A ̊, tracing several chemical elements including Mg and Fe. We apply these models to a large sample of z ~ 0.6 massive galaxies from the Sloan Digital Sky Survey (SDSS) – III / Baryon Oscillation Spectroscopic Survey (BOSS) to derive the ages of the UV bright populations. We find a subset of our indices to be non-degenerate between old and young UV ages allowing us to find evidence for old stars contributing to the UV, rather than new star formation. We find up to 47% of our working sample to contain a contribution from old UV bright stars, with those found to have higher contributing mass fractions being on average more massive and redder than those with lower mass fractions.