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Why do the oldest globular clusters set a minimum age for the Milky Way Galaxy?

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Whirlpool Galaxy (M51); NGC 5195 — Galaxy The Whirlpool Galaxy (left), also known as M51, an Sc galaxy accompanied by a small irregular companion galaxy, NGC 5195 (right).

Globular clusters serve as vital indicators of the age of the Milky Way Galaxy because they are the oldest objects within it, having formed before the galaxy’s material flattened into its current thin disk.123

Because massive stars evolve more rapidly than smaller ones, astronomers can estimate the age of a star cluster by examining its H-R diagram. In a young cluster, the main sequence is well populated, but in an old cluster, the heavier stars have already evolved away from the main sequence. The extent of this depopulation provides a reliable index of the cluster's age.2

By applying this method, the oldest globular clusters have been measured at approximately 12.5 billion years old. Since these clusters are among the first structures to have formed in the galaxy, their age establishes a minimum age for the Milky Way itself. Their status as the oldest objects is further supported by their low abundance of heavy elements compared to younger stars in the galactic plane.231