  Most previous efforts to calibrate how rotation and magnetic activity depend
on stellar age and mass have relied on observations of clusters, where
isochrones from stellar evolution models are used to determine the properties
of the ensemble. Asteroseismology employs similar models to measure the
properties of an individual star by matching its normal modes of oscillation,
yielding the stellar age and mass with high precision. We use 27 days of
photometry from the Transiting Exoplanet Survey Satellite to characterize
solar-like oscillations in the G8 subgiant of the 94 Aqr triple system. The
resulting stellar properties, when combined with a reanalysis of 35 yr of
activity measurements from the Mount Wilson HK project, allow us to probe the
evolution of rotation and magnetic activity in the system. The asteroseismic
age of the subgiant agrees with a stellar isochrone fit, but the rotation
period is much shorter than expected from standard models of angular momentum
evolution. We conclude that weakened magnetic braking may be needed to
reproduce the stellar properties, and that evolved subgiants in the hydrogen
shell-burning phase can reinvigorate large-scale dynamo action and briefly
sustain magnetic activity cycles before ascending the red giant branch.
