Aims: From the luminosity, effective temperature and age of the Hyades brown dwarf 2MASS J04183483+2131275 (2M0418), substellar evolutionary models predict a mass in the range 39-55 Jupiter masses (MJup) which is insufficient to produce any substantial lithium burning except for the very upper range >53 MJup. 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 2277 s each covering the range 6300-10 300 Å with a resolving power of R 500. Results: In the individual spectra, which span several months, we detect persistent unresolved Hα 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 ± 4 Å in 2M0418 (L5). Conclusions: We determine a lithium abundance of log N(Li) = 3.0 ± 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 MJup. 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 MJup and the age of the cluster to 580-775 Myr (1σ) based on the lithium depletion boundary method. Based on observations made with the Gran Telescopio de Canarias (GTC) installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma (programme GTC77-16B).The spectrum (FITS file) is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/615/L12