WordNet (r) 3.0 (2006):

infantilism
    n 1: an abnormal condition in which an older child or adult
         retains infantile characteristics
    2: infantile behavior in mature persons

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "infantilism":
   amentia, arrested development, babyishness, backwardness,
   blithering idiocy, childishness, cretinism, half-wittedness,
   idiocy, idiotism, imbecility, immaturity, insanity,
   mental defectiveness, mental deficiency, mental handicap,
   mental retardation, mongolianism, mongolism, mongoloid idiocy,
   moronism, moronity, profound idiocy, puerilism, puerility,
   retardation, retardment, simple-wittedness, simplemindedness,
   simpleness, simplicity, subnormality