Lithium abundance analysis
By measuring the stars' lithium (a sign of leftover fuel from their youth) scientists estimated that each star is about 9.0 billion years old... 5.7 +/- 0.5 Gyr... 4.0 +/- 1.0 Gyr.
Technical definition
Measurement and analysis of a star's lithium content to estimate its age, using the fact that lithium abundance decreases as stars get older.