infantilism

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

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