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It's harder to tell the age of an older star because they rotate more slowly and have smaller starspots.

It's easier to tell the age of a young star because they rotate more quickly and have larger starspots.

This artist's impression of a "cosmic clock" illustrates how astronomers have used stellar rotation to measure the ages of stars in a 2.5-billion-year-old star cluster. Their results, the latest success of gyrochronology, mark the first extension of such observations to stars with ages beyond 1 billion years, and toward the 4.6-billion-year age of the Sun. Being able to tell the ages of stars is the basis for understanding how astronomical phenomena involving stars and their companions unfold over time.