Age distributions of the globular clusters in 19 simulated MW-mass galaxies and their virial mass evolutions plotted below the respective age distributions in the same colors for each galaxy. The distributions are smoothed using an assumed uncertainty of 0.5 Gyr for the GC ages. The galaxies shown in the top left panels are the galaxies that reached half of their z = 0 virial mass at the earliest time, followed by the galaxies in the top right panel. The galaxies in the bottom right panel are those that reach half of the current virial mass the latest. The galaxy with the dashed virial mass evolution line has an issue with the merger tree, but the GC population is correct.