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We present optical and near-infrared adaptive optics (AO) imaging andspectroscopy of 13 ultracool (>M6) companions to late-type stars (K7-M4.5),most of which have recently been identified as candidate members of nearbyyoung moving groups (YMGs; 8-120 Myr) in the literature. The inferred masses ofthe companions (~10-100 Mjup) are highly sensitive to the ages of the primarystars so we critically examine the kinematic and spectroscopic properties ofeach system to distinguish bona fide YMG members from old field interlopers.2MASS J02155892-0929121 C is a new M7 substellar companion (40-60 Mjup) withclear spectroscopic signs of low gravity and hence youth. The primary, possiblya member of the ~40 Myr Tuc-Hor moving group, is visually resolved into threecomponents, making it a young low-mass quadruple system in a compact (<100 AU)configuration. In addition, Li 1 $\lambda$6708 absorption in theintermediate-gravity M7.5 companion 2MASS J15594729+4403595 B providesunambiguous evidence that it is young (<200 Myr) and resides below the hydrogenburning limit. Three new close-separation (<1") companions (2MASSJ06475229-2523304 B, PYC J11519+0731 B, and GJ 4378 Ab) orbit stars previouslyreported as candidate YMG members, but instead are likely old (>1 Gyr)tidally-locked spectroscopic binaries without convincing kinematic associationswith any known moving group. The high rate of false positives in the form ofold active stars with YMG-like kinematics underscores the importance of radialvelocity and parallax measurements to validate candidate young stars identifiedvia proper motion and activity selection alone. Finally, we spectroscopicallyconfirm the cool temperature and substellar nature of HD 23514 B, a recentlydiscovered M8 benchmark brown dwarf orbiting the dustiest-known member of thePleiades. [Abridged]

Planets Around Low-Mass Stars (PALMS). V. Age-Dating Low-Mass Companions to Members and Interlopers of Young Moving Groups