  The rotation rate, level of magnetic activity and surface lithium abundance
are age-dependent quantities in stars of about a solar mass and below. The
physical reasons for the evolution of these phenomena are qualitatively
understood, but accurate quantitative models remain dependent on empirical
calibration using the Sun and stars of known age, chiefly in clusters. In this
work I review the status of these "empirical age indicators", outlining the
astrophysics of their time dependence, describing the measurements, assessing
the precision (and accuracy) of age estimates when applied to individual stars,
and identifying their principle limitations in terms of the mass and age ranges
over which they are useful. Finally, I discuss the "lithium depletion boundary"
technique which, in contrast to the empirical methods, appears to provide
robust, almost model-independent ages that are both precise and accurate, but
which is only applicable to coeval groups of stars.
