Guessing your age might be a popular carnival game, but for astronomers it’s a real challenge to determine the ages of stars. Once a star like our sun has settled into steady nuclear fusion, or the mature phase of its life, it changes little for billions of years. One exception to that rule is the star’s rotation period—how quickly it spins. By measuring the rotation periods of hundreds of thousands of stars, NASA’s Nancy Grace Roman Space Telescope promises to bring new understandings of stellar populations in our Milky Way galaxy after it launches by May 2027.