We assume that all of the stars in a stellar cluster form at a single time, with the same chemical composition (out of the same materials found in local gas clouds). The highest mass stars inhabit the top end of the Main Sequence, and are the first to evolve into giant stars (and later into white dwarfs, neutron stars, and black holes). The lowest mass stars are found on the bottom end and can remain there for tens of billions of years, burning through their hydrogen fuel at a very slow pace.

The more stars that have evolved off of the Main Sequence, the older a cluster is.