A minimum age of the universe can be estimated directly by determining the
age of the oldest objects in the our Galaxy. These objects are the metal-poor
stars in the halo of the Milky Way. Recent work on nucleochronology finds that
the oldest stars are 15.2+/-3.7 Gyr old. White dwarf cooling curves have found
a minimum age for the oldest stars of 8 Gyr. Currently, the best estimate for
the age of the oldest stars is based upon the absolute magnitude of the main
sequence turn-off in globular clusters. The oldest globular clusters are
11.5+/-1.3 Gyr old, implying a minimum age of the universe of t_universe > 9.5
Gyr (95% confidence level).Comment: invited review to appear in Physics Report
