  We present a new volume-limited sample of L0-T8 dwarfs out to 25 pc defined
entirely by parallaxes, using our recent measurements from UKIRT/WFCAM along
with Gaia DR2 and literature parallaxes. With 369 members, our sample is the
largest parallax-defined volume-limited sample of L and T dwarfs to date,
yielding the most precise space densities for such objects. We find the local
L0-T8 dwarf population includes $5.5\%\pm1.3\%$ young objects ($\lesssim$200
Myr) and $2.6\%\pm1.6\%$ subdwarfs, as expected from recent studies favoring
representative ages $\lesssim$4 Gyr for the ultracool field population. This is
also the first volume-limited sample to comprehensively map the transition from
L to T dwarfs (spectral types $\approx$L8-T4). After removing binaries, we
identify a previously unrecognized, statistically significant (>4.4$\sigma$)
gap $\approx$0.5 mag wide in $(J-K)_{\rm MKO}$ colors in the L/T transition,
i.e., a lack of such objects in our volume-limited sample, implying a rapid
phase of atmospheric evolution. In contrast, the most successful models of the
L/T transition to date $-$ the "hybrid" models of Saumon & Marley (2008) $-$
predict a pile-up of objects at the same colors where we find a deficit,
demonstrating the challenge of modeling the atmospheres of cooling brown
dwarfs. Our sample illustrates the insights to come from even larger
parallax-selected samples from the upcoming Legacy Survey of Space and Time
(LSST) by the Vera Rubin Obsevatory.
