  In the past years, some extremely red brown dwarfs were found. They were
believed to have low surface gravity, but many of their spectral
characteristics were similar to those of high surface gravity brown dwarfs,
showing that youth (or low surface gravity) spectral characteristics are poorly
understood. We aim to test surface gravity indicators in late-M and early-L
brown dwarf spectra using data obtained with the X-shooter spectrograph at the
Very Large Telescope. We selected a benchmark sample of brown dwarfs members of
Chamaeleon\,I ($\sim$2 Myr), Upper Scorpius (5$-$10~Myr), Pleiades (132$\pm$27
Myr), and Praesepe (590$-$790 Myr) with well-constrained ages, and similar
metallicities. We provided a consistent spectral classification of the sample
in the optical and in the near-infrared. We measured the equivalent widths of
their alkali lines, finding that they have a moderate correlation with age,
especially for objects with spectral types M8 and later. We used spectral
indices defined in the literature to estimate surface gravity, finding that
their gravity assignment is accurate for 75\% of our sample. We investigated
the correlation between red colours and age, finding that after $\sim$10~Myr,
the colour does not change significantly for our sample {with spectral types
M6.0-L3.0}. In this case, red colours might be associated with circumstellar
disks, {ring structures, extinction, or viewing angle}. Finally, we calculated
the {bolometric luminosity}, {and $J$ and $K$ bolometric corrections} for our
sample. We found that six objects are overluminous compared to other members of
the same association. Those objects are also flagged as binary candidates by
the $Gaia$ survey.
