The Collaborative International Dictionary of English v.0.48:

Immature \Im`ma*ture"\, a. [L. immaturus; pref. im- not +
   maturus mature, ripe. See Mature.]
   1. Not mature; unripe; not arrived at perfection of full
      development; crude; unfinished; as, immature fruit;
      immature character; immature plans. "An ill-measured and
      immature counsel." --Bacon.
      [1913 Webster]

   2. Premature; untimely; too early; as, an immature death.
      [R.] --Jer. Taylor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

immature
    adj 1: characteristic of a lack of maturity; "immature behavior"
           [ant: mature]
    2: (used of living things especially persons) in an early period
       of life or development or growth; "young people" [syn:
       young, immature] [ant: old]
    3: not fully developed or mature; not ripe; "unripe fruit";
       "fried green tomatoes"; "green wood" [syn: green, unripe,
       unripened, immature] [ant: mature, ripe]
    4: not yet mature [ant: mature]
    5: (of birds) not yet having developed feathers; "a small
       unfledged sparrow on the window sill" [syn: unfledged,
       immature] [ant: fledged, mature]

Moby Thesaurus II by Grady Ward, 1.0:

152 Moby Thesaurus words for "immature":
   a novice at, a stranger to, adolescent, adulterated, amateur,
   amateurish, arrested, at half cock, babish, babyish, blemished,
   budding, callow, childish, childlike, dabbling, damaged, defective,
   deficient, dewy, dilettante, dilettantish, embryonic, erroneous,
   ever-new, evergreen, failing, fallible, faulty, firsthand,
   fledgling, found wanting, fresh, green, green as grass, growing,
   half-assed, half-baked, half-cocked, half-grown, hypoplastic,
   ignorant, ill-digested, impaired, imperfect, imprecise, impubic,
   impure, in arrear, in arrears, in default, in short supply,
   inaccurate, inadequate, incomplete, inexact, inexperienced, infant,
   infantile, infantine, ingenuous, innocent, intact, jejune, juicy,
   juvenile, lacking, maiden, maidenly, makeshift, mediocre, minor,
   missing, mixed, naive, needing, neoteric, nestling, new, new to,
   new-fledged, not perfect, off, original, part, partial, patchy,
   precocious, premature, pristine, puerile, raw, ripening,
   rudimentary, sappy, scant, scanty, sciolistic, scrappy,
   sempervirent, shallow, short, shy, sketchy, smattering, sophomoric,
   superficial, tender, unaccustomed to, unacquainted with, unadult,
   unbeaten, unconversant, unconversant with, underage,
   underdeveloped, underripe, undeveloped, uneven, unexperienced,
   unfamiliar with, unfinished, unfledged, unformed, ungrown,
   unhandled, uninitiated in, unlicked, unmatured, unmellowed,
   unperfected, unpracticed, unpracticed in, unripe, unseasoned,
   unskilled in, unsophisticated, unsound, unthorough, untouched,
   untried, untrodden, unused, unused to, unversed, unversed in,
   vernal, virgin, virginal, wanting, young, youthful