Gyrochronology is stellar age determination using the star's rotation rate along with other characteristics such as its mass and/or color (through its normal relation with the mass). It is considered effective for low mass stars, including the Sun. An early observed relationship, known as the Skumanich relationship or Skumanich law, for main sequence stars, is:
Ωe ∝ t-½
Subsequent studies have adjusted this exponent.
I believe stars are generally thought to slow their rotation quickly for a while (or during star formation), because few stars are seen with rotations above a certain range. After that, statistics suggest they slow their rotation slowly over a quite long time, and the slowing rates of many stars (in combination with the mass) can be correlated with other indications of stars' ages. The resulting correlation can be applied to other stars, e.g., more distant or lacking other clues to their age. The derived age has a considerable error bar but offers information when no other means is available.