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We present the characterization of CRTS J055255.7$-$004426 (=THOR 42), ayoung eclipsing binary comprising two pre-main sequence M dwarfs (combinedspectral type M3.5). This nearby (103 pc), short-period (0.859 d) system wasrecently proposed as a member of the $\sim$24 Myr-old 32 Orionis Moving Group.Using ground- and space-based photometry in combination with medium- andhigh-resolution spectroscopy, we model the light and radial velocity curves toderive precise system parameters. The resulting component masses and radii are$0.497\pm0.005$ and $0.205\pm0.002$ $\rm{M}_{\odot}$, and $0.659\pm0.003$ and$0.424\pm0.002$ $\rm{R}_{\odot}$, respectively. With mass and radiusuncertainties of $\sim$1 per cent and $\sim$0.5 per cent, respectively, THOR 42is one of the most precisely characterized pre-main sequence eclipsing binariesknown. Its systemic velocity, parallax, proper motion, colour-magnitude diagramplacement and enlarged radii are all consistent with membership in the 32 OriGroup. The system provides a unique opportunity to test pre-main sequenceevolutionary models at an age and mass range not well constrained byobservation. From the radius and mass measurements we derive ages of 22-26 Myrusing standard (non-magnetic) models, in excellent agreement with the age ofthe group. However, none of the models can simultaneously reproduce theobserved mass, radius, temperature and luminosity of the coeval components. Inparticular, their H-R diagram ages are 2-4 times younger and we infer masses$\sim$50 per cent smaller than the dynamical values.

THOR 42: A touchstone $\sim$24 Myr-old eclipsing binary spanning the fully-convective boundary