C. Maraston on "Stellar population Models"

Stellar population models are made up from models of individual stars by summing up the spectra of a distribution of stars of the same age and doing this at different timesteps, taking into account the evolution and lifetime of each type of stars.

This means that different types of stars dominate the light at different times, e.g. it is RGB stars that account for the bulk of light in old populations. To get this modelling right, it has to be calibrated against globular clusters, the stars in which are known to have formed together at some point. An alternative to this approach is using the fuel consumption theorem.

Newer models that take more physics (delayed RGB-onset) into account make the agreement with the spectra of high-z better and arrive at younger ages and smaller masses for these objects. Still, the age determinations are very insecure in some cases.