Opposite words for maturity:
minority, juvenility, infancy, immatureness, childhood, nonage, freshness, youngness, prime, youth, youthfulness, adolescence, toddlerhood, immaturity, babyhood.
Usage samples:
  1. The gills are not crowded, broad, free from the stem, white, then becoming flesh color with the maturity of the spores.
    "Studies of American Fungi. Mushrooms, Edible, Poisonous, etc.", George Francis Atkinson.
  2. The daughter of which Victoria was delivered, was at her maturity bestowed in marriage on me.
    "The Castle of Otranto", Horace Walpole.
  3. Augusta Maturity was silent awhile.
    "The Dwelling Place of Light, Complete", Winston Churchill Last Updated: March 5, 2009.
  4. It is not whilst held in leading- strings that they can be useful, or aught but burthensome: rear them kindly to maturity, and allow them the free exercise of their vast natural strength, and they would be to the parent country her truest and most valuable friends.
    "The Bushman Life in a New Country", Edward Wilson Landor.