Mass fractions of the young populations at which an old population is discernible from a young population. Galaxies with mass fractions fit below these lines have two observable populations, for those with mass fractions fit above the lines it is impossible to discern any older population. The limiting mass fraction where the older population becomes visible are shown with different colours, depending on the ages of the young population. At very young ages of the young population, all older populations are invisible. The solid vertical line marks the age limit of the young population in our fits. The solid horizontal lines are linear fits to the limiting mass fraction dependencies on the older population age. The peak at age  ~1.3 Gyr is a numerical artifact due to the limited age grid in the GALAXEV models. This does not affect our results as we use the solid, fitted lines for the measurements.