The Collaborative International Dictionary of English v.0.48:

Infantile \In"fan*tile\ (?; 277), a. [L. infantilis: cf. F.
   infantile. See Infant.]
   Of or pertaining to infancy, or to an infant; similar to, or
   characteristic of, an infant; childish; as, infantile
   behavior.
   [1913 Webster]

WordNet (r) 3.0 (2006):

infantile
    adj 1: indicating a lack of maturity; "childish tantrums";
           "infantile behavior" [syn: childish, infantile]
    2: of or relating to infants or infancy; "infantile paralysis"
    3: being or befitting or characteristic of an infant; "infantile
       games"

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "infantile":
   abecedarian, aboriginal, antenatal, autochthonous, babish, baby,
   babyish, beginning, budding, childish, childlike, creative,
   doll-like, dollish, elemental, elementary, embryonic, fetal,
   formative, foundational, fundamental, gestatory, immature, in arms,
   in diapers, in embryo, in its infancy, in nappies,
   in swaddling clothes, in the bud, in the cradle, inaugural,
   inceptive, inchoate, inchoative, incipient, incunabular, infant,
   infantine, initial, initiative, initiatory, introductory,
   inventive, juvenile, kittenish, nascent, natal, neonatal, newborn,
   original, parturient, postnatal, pregnant, prenatal, primal,
   primary, prime, primeval, primitive, primogenial, procreative,
   puerile, rudimental, rudimentary, ur