Massive early-type galaxies (ETGs) are mainly populated by old stars that dominate the optical spectral range. We analyse optical and near-ultraviolet (NUV) absorption lines from integrated spectra of massive ETGs to study its young stellar content, thanks to the extreme sensitivity of the NUV to young stars. We find that young stellar populations are ubiquitous in massive ETGs at z∼0.4, measuring average mass fractions of 0.5% in stars formed within the last 2 Gyr of their evolution. This fraction decreases with galaxy stellar mass, consistent with a down-sizing scenario. We also study spectra of 6 massive brightest cluster galaxies (BCGs) to find that the young stars are concentrated in their central regions (<0.8 kpc), decreasing outwards to virtually no young stars beyond 2 kpc.