nonage
Synonym discussion
Sense: noun/1
For noun/1, nonage is a formal and rather old-fashioned word for the legal condition of still being under full adult age. Minority is the nearest formal equivalent and is still more common in legal writing. Youth is much broader and usually refers to the years of being young, so it fits sense 2 better than this legal sense. Immaturity is different again: it describes childish behavior or weak judgment, which matches sense 3 rather than legal status. In a will or court record, writers may mention property held during a person's nonage. In ordinary conversation, people would normally say someone is underage instead of using nonage.
Etymology
Nonage came into English from Anglo-French and Middle French nonage, literally meaning 'not of age'. It combines non- ('not') with age, so its earliest use was for the condition of not yet being old enough in law to act as an adult. From that legal meaning, writers later extended it to a person's younger years in general and, by another extension, to the immaturity often linked with youth.