The Collaborative International Dictionary of English v.0.48:
Immatureness \Im`ma*ture"ness\, n. The state or quality of being immature; immaturity. --Boyle. [1913 Webster]WordNet (r) 3.0 (2006):
immatureness n 1: not having reached maturity [syn: immaturity, immatureness] [ant: matureness, maturity]