Source : Webster's Revised Unabridged Dictionary (1913)

Immaturely \Im`ma*ture"ly\, adv.
   In an immature manner. --Warburion.

Source : WordNet®

immaturely
     adv : in an immature manner; "his teenage son still behaves very
           immaturely" [syn: {jejunely}] [ant: {maturely}]