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We report the discovery of two transiting brown dwarfs (BDs), TOI-811b andTOI-852b, from NASA's Transiting Exoplanet Survey Satellite mission. These twotransiting BDs have similar masses, but very different radii and ages. Theirhost stars have similar masses, effective temperatures, and metallicities. Theyounger and larger transiting BD is TOI-811b at a mass of $M_b = 55.3 \pm3.2{\rm M_J}$ and radius of $R_b = 1.35 \pm 0.09{\rm R_J}$ and it orbits itshost star in a period of $P = 25.16551 \pm 0.00004$ days. Its age of$93^{+61}_{-29}$ Myr, which we derive from an application of gyrochronology toits host star, is why this BD's radius is relatively large, not heating fromits host star since this BD orbits at a longer orbital period than most knowntransiting BDs. This constraint on the youth of TOI-811b allows us to testsubstellar mass-radius isochrones where the radius of BDs changes rapidly withage. TOI-852b is a much older (4.0 Gyr from stellar isochrone models of thehost star) and smaller transiting BD at a mass of $M_b = 53.7 \pm 1.3{\rmM_J}$, a radius of $R_b = 0.75 \pm 0.03{\rm R_J}$, and an orbital period of $P= 4.94561 \pm 0.00008$ days. TOI-852b joins the likes of other old transitingBDs that trace out the oldest substellar mass-radius isochrones wherecontraction of the BD's radius asymptotically slows. Both host stars have amass of $M_\star = 1.32{\rm M_\odot}\pm0.05$ and differ in their radii, $T_{\rmeff}$, and [Fe/H] with TOI-811 having $R_\star=1.27\pm0.09{\rm R_\odot}$,$T_{\rm eff} = 6107 \pm 77$K, and $\rm [Fe/H] = +0.40 \pm 0.09$ and TOI-852having $R_\star=1.71\pm0.04{\rm R_\odot}$, $T_{\rm eff} = 5768 \pm 84$K, and$\rm [Fe/H] = +0.33 \pm 0.09$. We take this opportunity to examine how TOI-811band TOI-852b serve as test points for young and old substellar isochrones,respectively.

TOI-811b and TOI-852b: New transiting brown dwarfs with similar masses and very different radii and ages from the TESS mission