Isochrones of radii (R_J) vs. mass (M_⊙) for 2MASS J0746+20AB, for ages of ~1 (log 9.0; red), ~1.25 (log 9.1; blue) and ~1.5 Gyr (log 9.2; green), derived in this work by using the evolutionary models of Chabrier et al. (2000). The errors associated with the mass and radii estimates are based on the constraints of the Chabrier et al. (2000) models for a given age, as outlined in Sect. 4. We also include evolutionary tracks for ages of ~150–500 Myr (log  8.2–8.7 years) as predicted by Bouy et al. (2004). These are shown by the grey dashed lines. Based on the work this paper, the binary appears to be a much older age than the range predicted by Bouy et al. (2004), which require much larger radii for both components.