What specific feature on the H-R diagram is the key to determining a star cluster's age?

The main-sequence turnoff point

When analyzing a star cluster, all members share a common formation time, making them excellent age indicators. The key technique relies on identifying the main-sequence turnoff point on the H-R diagram. This specific location marks the transition where the most massive stars within that cluster, having burned their core hydrogen fastest, are just beginning to exhaust their fuel and evolve away from the main sequence towards regions like the red giant branch. By calculating the theoretical lifespan corresponding to the mass of a star found exactly at this turnoff point, astronomers accurately date the entire population that shares that common age.