Using a large sample of optical spectra of late-type dwarfs, we identify a subset of late-M through L field dwarfs that, because of the presence of low-gravity features in their spectra, are believed to be unusually young. From a combined sample of 303 field L dwarfs, we find observationally that 7.6+/-1.6% are younger than 100Myr. This percentage is in agreement with theoretical predictions once observing biases are taken into account. We find that these young L dwarfs tend to fall in the southern hemisphere (DEC<0{deg}) and may be previously unrecognized, low-mass members of nearby, young associations like Tucana-Horologium, TW Hydrae, {beta} Pictoris, and AB Doradus. We use a homogeneously observed sample of ~150 optical spectra to examine lithium strength as a function of L/T spectral type and further corroborate the trends noted by Kirkpatrick and coworkers. We use our low-gravity spectra to investigate lithium strength as a function of age. The data weakly suggest that for early- to mid-L dwarfs the line strength reaches a maximum for a fewx100Myr, whereas for much older (few Gyr) and much younger (=M7) with spectroscopic signatures of low gravity (Youth))

Cone search capability for table J/ApJ/689/1295/table7 (Log of additional spectroscopic observations for objects shown in Figs. 7-13)

Cone search capability for table J/ApJ/689/1295/table8 (Lithium equivalent width measurements for L0, L2, L4, and L6.5 dwarfs)