The Collaborative International Dictionary of English v.0.48:

Immaturity \Im`ma*tu"ri*ty\, n. [L. immaturitas: cf. F.
   immaturit['e].]
   The state or quality of being immature or not fully
   developed; unripeness; incompleteness.
   [1913 Webster]

         When the world has outgrown its intellectual
         immaturity.                              --Caird.
   [1913 Webster]

WordNet (r) 3.0 (2006):

immaturity
    n 1: not having reached maturity [syn: immaturity,
         immatureness] [ant: matureness, maturity]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "immaturity":
   adulteration, amateurishness, amateurism, arrestment, babyishness,
   brand-newness, callowness, childishness, coarseness, crudeness,
   crudity, defectibility, defectiveness, deficiency, dewiness,
   erroneousness, fallibility, faultiness, freshness,
   gloss of novelty, greenness, ignorance, immatureness, impairment,
   imperfection, imprudence, impurity, inaccuracy, inadequacy,
   inadequateness, inadvisability, inattention, incompleteness,
   incompletion, inconsideration, indiscreetness, indiscretion,
   inexactitude, inexactness, inexpedience, inexperience, infancy,
   infantilism, injudiciousness, intactness, irrationality, juiciness,
   juniority, lack, lateness, maidenhood, mediocrity, minority,
   mint condition, newbornness, newfangledness, newfangleness,
   newness, nonage, nondevelopment, novelness, novelty, nowness,
   originality, oversimplicity, oversimplification, patchiness,
   pompousness, presentness, pristineness, puerilism, puerility,
   rawness, reasonlessness, recency, recentness, recklessness,
   reductionism, roughness, rudeness, sappiness, scrappiness,
   senselessness, short measure, short weight, shortage, shortcoming,
   simplism, sketchiness, strangeness, stuffiness, the rough,
   thoughtlessness, unaccustomedness, unacquaintance,
   unacquaintedness, uncommonness, uncultivation, underdevelopment,
   undevelopment, unevenness, unexperiencedness, unfamiliarity,
   unfinish, unfinishedness, unfledgedness, unintelligence,
   unperfectedness, unpracticedness, unprofessionalism,
   unprofessionalness, unreason, unreasonableness, unrefinement,
   unripeness, unsensibleness, unsoundness, unthoughtfulness,
   unusualness, unwisdom, unwiseness, virginity, want, witlessness