  The nearby (d = 12 pc) M8 dwarf star TRAPPIST-1 (2MASS J23062928-0502285)
hosts a compact system of at least seven exoplanets with sizes similar to
Earth. Given its importance for testing planet formation and evolution
theories, and for assessing the prospects for habitability among Earth-size
exoplanets orbiting the most common type of star in the Galaxy, we present a
comprehensive assessment of the age of this system. We collate empirical age
constraints based on the color-absolute magnitude diagram, average density,
lithium absorption, surface gravity features, metallicity, kinematics,
rotation, and magnetic activity; and conclude that TRAPPIST-1 is a transitional
thin/thick disk star with an age of 7.6$\pm$2.2 Gyr. The star's color-magnitude
position is consistent with it being slightly metal-rich ([Fe/H] $\simeq$
+0.06), in line with its previously reported near-infrared spectroscopic
metallicity; and it has a radius (R = 0.121$\pm$0.003 R$_{\odot}$) that is
larger by 8-14% compared to solar-metallicity evolutionary models. We discuss
some implications of the old age of this system with regard to the stability
and habitability of its planets.
