nonage

1. The period during which one is legally underage.
2. A period of immaturity: "The bravest achievements were always accomplished in the nonage of a nation" (Thomas Paine).

[Middle English nounage, from Anglo-Norman, variant of Old French nonaage : non-, non- + aage, age; see age.]

noun

The state or period of being under legal age: minority. Law infancy. See law, youth/age/maturity.

Infancy or minority; lack of requisite legal age.

Nonage entails various contractual disabilities and is a ground for annulment in some jurisdictions.

The noun has one meaning:

Meaning #1: any age prior to the legal age
  Synonym: minority