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We present an analysis of 23 L dwarfs whose optical spectra display unusualfeatures. Twenty-one were uncovered during our search for nearby, late-typeobjects using the Two Micron All-Sky Survey while two were identified in theliterature. The unusual spectral features, notably weak FeH molecularabsorption and weak Na I and K I doublets, are attributable to low-gravity andindicate that these L dwarfs are young, low-mass brown dwarfs. We use thesedata to expand the spectral classification scheme for L0 to L5-type dwarfs toinclude three gravity classes. Most of the low-gravity L dwarfs have southerlydeclinations and distance estimates within 60 pc. Their implied youth, on-skydistribution, and distances suggest that they are members of nearby,intermediate-age (~10-100 Myr), loose associations such as the Beta Pictorismoving group, the Tucana/Horologium association, and the AB Doradus movinggroup. At an age of 30 Myr and with effective temperatures from 1500 to 2400 K,evolutionary models predict masses of 11-30 M_Jupiter for these objects. Oneobject, 2M 0355+11, with J-K_s=2.52+/-0.03, is the reddest L dwarf found in thefield and its late spectral type and spectral features indicative of a very lowgravity suggest it might also be the lowest-mass field L dwarf. However, beforeages and masses can be confidently adopted for any of these low-gravity Ldwarfs, additional kinematic observations are needed to confirm clustermembership.

Young L Dwarfs Identified in the Field: A Preliminary Low-Gravity, Optical Spectral Sequence from L0 to L5