  To understand how planetary spin evolves and traces planet formation
processes, we measure rotational line broadening in eight planetary-mass
objects (PMOs) of various ages (1--800 Myr) using near-infrared high-resolution
spectra from NIRSPEC/Keck. Combining these with published rotation rates, we
compile 27 PMO spin velocities, 16 of which derive from our NIRSPEC/Keck
program. Our data are consistent with spin velocities $v$ scaling with
planetary radius $R$ as $v \propto 1/R$. We conclude that spin angular momentum
is conserved as objects cool and contract over the sampled age range. The PMOs
in our sample spin at rates that are approximately an order of magnitude below
their break-up values, consistent with the hypothesis that they were spun down
by magnetized circum-PMO disks (CPDs) during the formation era at ages
$\lesssim$ a few Myr. There is a factor of 4--5 variation in spin velocity that
has yet to be understood theoretically. It also remains to be seen whether spin
evolves on timescales $\gtrsim$ 1 Gyr for PMOs, as it does for stars and
high-mass brown dwarfs emitting magnetized winds.
