The age of an individual star cannot be measured, only estimated through
mostly model-dependent or empirical methods, and no single method works well
for a broad range of stellar types or for a full range in age. This review
presents a summary of the available techniques for age-dating stars and
ensembles of stars, their realms of applicability, and their strengths and
weaknesses. My emphasis is on low-mass stars because they are present from all
epochs of star formation in the Galaxy and because they present both special
opportunities and problems. The ages of open clusters are important for
understanding the limitations of stellar models and for calibrating empirical
age indicators. For individual stars, a hierarchy of quality for the available
age-dating methods is described. Although our present ability to determine the
ages of even the nearest stars is mediocre, the next few years hold great
promise as asteroseismology probes beyond stellar surfaces and starts to
provide precise interior properties of stars and as models continue to improve
when stressed by better observations.Comment: To appear in the 2010 volume of Annual Reviews of Astronomy and
Astrophysics
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