Stellar ages as a function of observed redshift of log(M_⋆/M_⊙) > 10.5 quiescent galaxies. Adapted from Onodera et al. (2015). Here mass-weighted ages from the literature differ from t₅₀ in that they do not specifically refer to the lookback time at which 50% of M_⋆ is formed. The four targets selected from Schreiber et al. (2018) correspond to those recently followed up by Esdaile et al. (2021). Solid grey lines show, from thin to thick, the age of simple stellar populations made at a z_form from 1.5 to 5. The dotted red line marks the age of the Universe as a function of redshift.