Final answer:
The oldest globular cluster can be determined by looking at the main-sequence turnoff point on an H-R diagram.
Step-by-step explanation:
The main-sequence turnoff point in an H-R diagram can be used to determine the age of a cluster. In globular clusters, which are the oldest clusters, there are no luminous blue stars at all. The turnoff point represents where stars begin to leave the main sequence due to their evolution. So, the globular cluster with the earliest main-sequence turnoff indicates the oldest cluster.