Rotation Rate Could Pin Down Age of Stars: Scientific American: One feature, though, does change: stars spin more slowly as they grow old. “And so we can use the spin rate, the rotation rate of a star, as a clock to measure its age,” Meibom said.
But first someone has to paint the numbers on the face of that clock. Researchers have already pinned down the relation between rotation and age for very young stars. So Meibom and his colleagues are measuring the rotation rates of older stars.