The Collaborative International Dictionary of English v.0.48:
Maturely \Ma*ture"ly\, adv. 1. In a mature manner; with ripeness; completely. [1913 Webster] 2. With caution; deliberately. --Dryden. [1913 Webster] 3. Early; soon. [A Latinism, little used] --Bentley. [1913 Webster]WordNet (r) 3.0 (2006):
maturely adv 1: in a mature manner; "she acts very maturely for her age" [ant: immaturely, jejunely]