Antonym of underage or minority

Adult (or "legal adult" to be specific) — TFD

(legal definition) a person who has attained the age of legal majority.

A person who by virtue of attaining a certain age, generally eighteen, is regarded in the eyes of the law as being able to manage his or her own affairs.

The age specified by law, called the legal age of majority, indicates that a person acquires full legal capacity to be bound by various documents, such as contracts and deeds, that he or she makes with others and to commit other legal acts such as voting in elections and entering marriage.

The age at which a person becomes an adult varies from state to state and often varies within a state, depending upon the nature of the action taken by the person. Thus, a person wishing to obtain a license to operate a motor vehicle may be considered an adult at age sixteen, but may not reach adulthood until age eighteen for purposes of marriage, or age twenty-one for purposes of purchasing intoxicating liquors.

Anyone who has not reached the age of adulthood is legally considered an infant.

Age of majority — Wikipedia

Age of majority should not be confused with the age of sexual consent, marriageable age, school leaving age, drinking age, driving age, voting age, smoking age, etc., which each may be independent of, and sometimes set at a different age from, the age of majority

You can use the expression,

be/come of age:

• to reach the age when you are legally recognized as an adult.

(Cambridge Dictionary)