  The Upper Scorpius OB association is the nearest region of recent massive
star formation and thus an important benchmark for investigations concerning
stellar evolution and planet formation timescales. We present nine EBs in Upper
Scorpius, three of which are newly reported here and all of which were
discovered from K2 photometry. Joint fitting of the eclipse photometry and
radial velocities from newly acquired Keck-I/HIRES spectra yields precise
masses and radii for those systems that are spectroscopically double-lined. The
binary orbital periods in our sample range from 0.6-100 days, with total masses
ranging from 0.2-8 $M_\odot$. At least 33% of the EBs reside in hierarchical
multiples, including two triples and one quadruple. We use these EBs to develop
an empirical mass-radius relation for pre-main-sequence stars, and to evaluate
the predictions of widely-used stellar evolutionary models. We report evidence
for an age of 5-7 Myr which is self-consistent in the mass range of 0.3-5
$M_\odot$ and based on the fundamentally-determined masses and radii of
eclipsing binaries (EBs). Evolutionary models including the effects of magnetic
fields imply an age of 9-10 Myr. Our results are consistent with previous
studies that indicate many models systematically underestimate the masses of
low-mass stars by 20-60% based on H-R diagram analyses. We also consider the
dynamical states of several binaries and compare with expectations from tidal
dissipation theories. Finally, we identify RIK 72 b as a long-period transiting
brown dwarf ($M = 59.2 \pm 6.8\ M_\mathrm{Jup}$, $R = 3.10 \pm 0.31\
R_\mathrm{Jup}$, $P \approx 97.8$ days) and an ideal benchmark for brown dwarf
cooling models at 5-10 Myr.
