These estimates depend on comparing the cluster star brightnesses to similar and closer stars of known distance. The latest data from the Hipparcos satellite shows that these close stars are actually somewhat farther (and brighter) than previously estimated. This means that the globular clusters are farther and younger than thought. Astronomers using Hipparcos data now estimate globular cluster ages as around 11.5 billion years. This is younger than the oldest estimates of the age of the universe. The “Universe age versus star age” problem seems to be going away.
For detailed information on the latest star age and distance estimates, follow the following links:
Star Age Page
The Hipparcos website