The Collaborative International Dictionary of English v.0.48:

Nymphet \Nymph"et\, n.
   A little or young nymph. [Poetic] "The nymphets sporting
   there." --Drayton.
   [1913 Webster] Nymphic

WordNet (r) 3.0 (2006):

nymphet
    n 1: a sexually attractive young woman

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "nymphet":
   Atlantides, Hyades, Jezebel, Napaea, Pleiades, babe, baby,
   bad woman, baggage, bitch, broad, chick, chippy, clitoromaniac,
   cocotte, colleen, cutie, dame, damoiselle, damsel, demoiselle,
   doll, drab, dryad, easy lay, easy woman, filly, floozy,
   flower nymph, frail, frail sister, gal, girl, girlie, glen nymph,
   grisette, hamadryad, harridan, heifer, hoyden, hussy,
   hysteromaniac, jade, jeune fille, jill, junior miss, lass, lassie,
   limoniad, little missy, loose woman, mademoiselle, maid, maiden,
   miss, missy, mountain nymph, nymph, nymphlin, nympho, nymphomaniac,
   oread, pickup, piece, quean, romp, schoolgirl, schoolmaid,
   schoolmiss, skirt, slip, slut, strumpet, subdeb, subdebutante,
   subteen, subteener, tart, teenybopper, tomato, tomboy, tree nymph,
   trollop, trull, uteromaniac, vila, virgin, wanton, wench, whore,
   willi, wood nymph, young creature, young thing