  The rotation rates of main-sequence stars slow over time as they gradually
lose angular momentum to their magnetized stellar winds. The rate of angular
momentum loss depends on the strength and morphology of the magnetic field, the
mass-loss rate, and the stellar rotation period, mass, and radius. Previous
observations suggested a shift in magnetic morphology between two F-type stars
with similar rotation rates but very different ages (88 Leo and rho CrB). In
this Letter, we identify a comparable transition in an evolutionary sequence of
solar analogs with ages between 2-7 Gyr. We present new spectropolarimetry of
18 Sco and 16 Cyg A & B from the Large Binocular Telescope, and we reanalyze
previously published Zeeman Doppler images of HD 76151 and 18 Sco, providing
additional constraints on the nature and timing of this transition. We combine
archival X-ray observations with updated distances from Gaia to estimate
mass-loss rates, and we adopt precise stellar properties from asteroseismology
and other sources. We then calculate the wind braking torque for each star in
the evolutionary sequence, demonstrating that the rate of angular momentum loss
drops by more than an order of magnitude between the ages of HD 76151 and 18
Sco (2.6-3.7 Gyr) and continues to decrease modestly to the age of 16 Cyg A & B
(7 Gyr). We suggest that this magnetic transition may represent a disruption of
the global dynamo arising from weaker differential rotation, and we outline
plans to probe this phenomenon in additional stars spanning a wide range of
spectral types.
