GC ages and virial mass evolutions of two selected modeled MW-mass galaxies compared to the observed GC ages in the MW. Top: cumulative GC age distributions of the MW from Usher et al. (2019) (blue) and of two modeled GC populations in simulated MW-mass galaxies from Valenzuela et al. (2021) (red and orange). Middle: age distributions of the same three GC populations as in the top panel. The smooth distributions are smoothed using an assumed uncertainty of 0.75 Gyr for the GC ages. These are computed through a summation over normal distributions with the respective ages as the means and their uncertainties as the standard deviations. For the sample from Usher et al. (2019), the distribution was scaled by a factor of four to be comparable to the modeled populations. Bottom: dark matter virial mass evolution of the two modeled galaxies, showing their accretion histories. The shaded region between 8 Gyr and 11 Gyr indicates the estimated time of the GSE merger (Belokurov et al. 2018; Helmi et al. 2018).