What analytical feature on the H-R diagram reveals a star cluster's age?

The Main Sequence Turnoff Point where stars cease burning core hydrogen.

When analyzing star clusters, plotting their members on the H-R diagram creates a distinct pattern defined by the Main Sequence. The most sensitive indicator of the cluster's age is the Main Sequence Turnoff Point. This point marks the location where the most massive stars in the cluster have exhausted their core hydrogen fuel and have begun evolving off the Main Sequence into giants or supergiants. Since massive stars evolve much faster than low-mass stars, the point at which stars peel away directly corresponds to the cluster's total age. If only lower-mass stars remain on the Main Sequence, the cluster is old; if higher-mass stars are still present, the cluster is young.