Globular clusters are spherical collections of stars bound to each other by their mutual gravity. They are old, relics of the early years of the Universe, with ages of typically 12-13 billion years, and we know of roughly 150 globular clusters in the Milky Way.
Yet, like many humans, these clusters are still young at heart. Some are aging faster than others and that discovery has led to a way to measure the rate of aging.
Star clusters form in a short period of time, meaning that all the stars within them tend to have roughly the same age. Because bright, high-mass stars burn up their fuel quite quickly, and globular clusters are very old, there should only be low-mass stars still shining within them.