  We aim to provide a detailed characterization of near-infrared spectra for
young LT brown dwarfs, including robust spectral typing, calibrating spectral
indices, identifying possible binaries, and selecting suitable spectral
standards. We processed and analyzed archival spectra from VLT/X-shooter for a
sample of 56 dwarfs with ages between 10 and 600 Myr and spectral types between
late-M and mid-T. We re-determine spectral types by comparing them with a set
of literature templates and assess a large range of spectral indices,
calibrated using a specifically designed literature sample. We are able to
identify 15 spectral indices that are useful for spectral typing for specific
spectral ranges discussed here and provide the scaling relations with spectral
types. Moreover, we also identify 6 spectral indices which can be used to
separate young L dwarfs from the field ones. The equivalent-widths of the
alkali lines show a correlation with age, increasing towards the objects with
higher surface gravity. From our sample, we confirm 3 that are likely to be
binaries by their anomalous spectra that appear to be better fitted by a
combination of spectral types. Finally, we identify 12 objects as preliminary
near-infrared spectral standards for young LT dwarfs. This paper presents a
significant step toward understanding the spectral sequence and properties of
young L and T dwarfs. The relations and standards provided here will be useful
for future spectroscopic work on young brown dwarfs and giant planets.
