The Collaborative International Dictionary of English v.0.48:

Nonage \Non"age\, n. [LL. nonagium, from L. nonus ninth, novem
   nine.] (Eccl.)
   The ninth part of movable goods, formerly payable to the
   clergy on the death of persons in their parishes. --Mozley &
   W.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Nonage \Non"age\, n. [Pref. non- + age.]
   Time of life before a person becomes of age; legal
   immaturity; minority.
   [1913 Webster]

         The human mind . . . was still in its nonage.
                                                  --Coleridge.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nonage
    n 1: any age prior to the legal age [syn: minority, nonage]
         [ant: legal age, majority]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "nonage":
   Nina from Carolina, callowness, dewiness, ennead, enneastyle,
   freshness, greenness, immaturity, inexperience, infancy, juiciness,
   juniority, minority, nine, nonagon, nonuplet, novena, rawness,
   sappiness, undevelopment, unripeness