Evolutionary tracks of low-mass stars and brown dwarfs from the BCAH15 models. Hydrogen-burning stars reach stable luminosities after a certain contraction time, while brown dwarfs continue to cool. The red square indicates the luminosity (1σ) and age (30–40 Myr) range of HD 24121B. The blue squares show the companions at the hydrogen-burning limit from the Gaia ultracool dwarf companion catalogue, which includes objects within 100 pc (Baig et al. 2024). HD 24121B is located at the hydrogen-burning limit between low-mass stars and brown dwarfs, and it is much younger than the other companions.