  We present a uniform forward-modeling analysis of 90 late-M and L dwarfs in
nearby young (~$10-200$ Myr) moving groups, the Pleiades, and the Hyades using
low-resolution ($R\approx150$) near-infrared ($0.9-2.4$ $\mathrm{\mu m}$)
spectra and the BT-Settl model atmospheres. We derive the objects' effective
temperatures, surface gravities, radii, and masses by comparing our spectra to
the models using a Bayesian framework with nested sampling and calculate the
same parameters using evolutionary models. Assuming the evolutionary-based
parameters are more robust, our spectroscopically inferred parameters from
BT-Settl exhibit two types of systematic behavior for objects near the M-L
spectral type boundary. Several are clustered around $T_\mathrm{eff} \approx
1800$ K and $\log g\approx5.5$ dex, implying impossibly large masses
($150-1400$ $M_\mathrm{Jup}$), while others are clustered around
$T_\mathrm{eff}\gtrsim3000$ K and $\log g\lesssim3.0$ dex, implying
non-physical low masses and unreasonably young ages. We find the fitted
BT-Settl model spectra tend to overpredict the peak $J$ and $H$-band flux for
objects located near the M-L boundary, suggesting the dust content included in
the model atmospheres is insufficient to match the observations. By adding an
interstellar medium-like reddening law to the BT-Settl model spectra, we find
the fits between models and observed spectra are greatly improved, with the
largest reddening coefficients occurring at the M-L transition. This work
delivers a systematic examination of the BT-Settl model atmospheres and
constitutes the largest spectral analysis of benchmark late-M and L-type brown
dwarfs to date.
