Context :

Aims : From the luminosity , effective temperature , and age of the Hyades brown dwarf 2MASS J04183483 + 2131275 ( 2M0418 ) , sub-stellar evolutionary models predict a mass in the range 39 - 55 Jupiter masses ( M _ { Jup } ) which is insufficient to produce any substantial lithium burning except for the very upper range > 53 M _ { Jup } .
Our goal is to measure the abundance of lithium in this object , test the consistency between models and observations and refine constraints on the mass and age of the object .

Methods : We used the 10.4-m Gran Telescopio Canarias ( GTC ) with its low-dispersion optical spectrograph to obtain ten spectra of 2277s each covering the range 6300–10300 Å with a resolving power of R \sim 500 .

Results : In the individual spectra , which span several months , we detect persistent unresolved H \alpha in emission with pseudo equivalent widths ( pEW ) in the range 45–150 Å and absorption lines of various alkalis with the typical strengths found in objects of L5 spectral type .
The lithium resonance line at 6707.8 Å is detected with pEW of 18 \pm 4 Å in 2M0418 ( L5 ) .

Conclusions : We determine a lithium abundance of \log N ( Li ) = 3.0 \pm 0.4 dex consistent with a minimum preservation of 90 % of this element which confirms 2M0418 as a brown dwarf with a maximum mass of 52 M _ { Jup } .
We infer a maximum age for the Hyades of 775 Myr from a comparison with the BHAC15 models .
Combining recent results from the literature with our study , we constrain the mass of 2M0418 to 45–52 M _ { Jup } and the age of the cluster to 580–775 Myr ( 1 \sigma ) based on the lithium depletion boundary method .