Which type of cluster is considered "old"?

Globular clusters are considered "old" because they contain some of the earliest stars formed in the universe. These clusters typically consist of hundreds of thousands to millions of stars that are densely packed together and are primarily made up of Population II stars, which are older, low-metallicity stars. The formation of globular clusters dates back to the early stages of galaxy formation, indicating that they have survived for billions of years, while the stars within them have aged significantly.

In contrast, other types of clusters, such as open clusters, are generally younger and contain more massive stars that have shorter lifespans. Open clusters form in regions with recent star formation and can still contain young stars alongside older ones. Protostellar clusters are in an even earlier phase of stellar development, where stars are still in the process of forming. Thus, the age and composition of globular clusters distinguish them as the "old" clusters in astronomical terms.