From Ethan Siegel:
However, the uncertainties are much smaller when we look at large collections of stars. The collections of stars that form within a galaxy like the Milky Way — open star clusters — typically contain a few thousand stars and only last a few hundred million years. The gravitational interactions between these stars eventually cause them to fly apart. While a small percentage last a billion years or even a few billion years, we have no known open star clusters that are even as old as our own Solar System.
Globular clusters, however, are larger, more massive, and more isolated, found throughout the halo of the Milky Way (and most large galaxies). When we observe them, we can measure the colors and brightnesses of many of the stars inside, enabling us — so long as we understand how stars work and evolve — to determine the ages of these star clusters. Although there are uncertainties here as well, there is a large population of globular clusters, even within the Milky Way alone, with ages of 12 billion years or more.
Read the whole thing.