Brown dwarfs — stellar objects unable to sustain hydrogen fusion in their cores because of their low masses — continuously cool over their lifetimes. Evolution models have been created to reproduce this behaviour, and to allow mass and age determination using their luminosity, temperatures, spectral types and other parameters. However, these models have not yet been fully validated or calibrated with observations. During a commissioning run of the SPECULOOS telescopes, we serendipitously discovered a rare double-line eclipsing binary, a member of the 45 Myr-old moving group Argus. This discovery permitted us to determine the masses, radii and ages of the brown dwarfs, and with their luminosities make a comparison to evolution models. The models reproduce these measurements remarkably well, although a measured offset in luminosity could result in systematic underestimation of brown dwarf masses by 20 to 30%. Calibrating these models is necessary as they are also used to infer the masses of young, directly imaged exoplanets such as those found at the VLT.