  A variation of Newton's constant $G$ over cosmological time scales would
modify the main-squence time of globular cluster (GC) stars. We have calculated
the evolution of low-mass stars typical for GCs both for standard non-varying
$G$ and under the assumption of a linear variation of $G$. The age of the
isochrones resulting from the latter models then was chosen such that the
isochrones mimicked the standard ones at the turnoff. Assuming that the true
age of GCs is between $8$ and $20\,\rm Gyr$, and because their apparent age is
between $14$ and $18\,\rm Gyr$, we find that today $-35{\times}10^{-12}\,{\rm
yr}^{-1}\la\dot{G}/G\la 7{\times}10^{-12}\,{\rm yr}^{-1}$. The upper limit
(gravity weaker in the past) is competitive with direct present-day bounds from
celestial mechanics. Within independently determined $\dot{G}/G$ limits a
time-varying $G$ as an explanation for the discrepancy between the cosmic
expansion age and the apparent GC ages is conceivable.
