  We report new observations of the unusually active, high proper motion L5e
dwarf 2MASS J13153094-2649513. Optical spectroscopy with Magellan/MagE reveals
persistent nonthermal emission, with narrow H I Balmer, Na I and K I lines all
observed in emission. Low-resolution near-infrared spectroscopy with IRTF/SpeX
indicates the presence of a low-temperature companion, which is resolved
through multi-epoch laser guide star adaptive optics imaging at Keck. The
comoving companion is separated by 338 \pm 4 mas, and its relative brightness
(\Delta K_s = 5.09 \pm 0.10) makes this system the second most extreme flux
ratio very low-mass binary identified to date. Resolved near-infrared
spectroscopy with Keck/OSIRIS identifies this companion as a T7 dwarf. The
absence of Li I absorption in combined-light optical spectroscopy constrains
the system age to >~0.8-1.0 Gyr, while the system's kinematics and unusually
low mass ratio (M_2/M_1 = 0.3-0.6) suggests that it is even older. A coevality
test of the components also indicates an older age, but reveals discrepancies
between evolutionary and atmosphere model fits of the secondary which are
likely attributable to poor reproduction of its near-infrared spectrum. With a
projected separation of 6.6 \pm 0.9 AU, the 2MASS J13153094-2649513 system is
too widely separated for mass exchange or magnetospheric interactions to be
powering its persistent nonthermal emission. Rather, the emission is probably
chromospheric in nature, signaling an inversion in the age-activity relation in
which strong magnetic fields are maintained by relatively old and massive
ultracool dwarfs.
