of age
to be fully developed, to be mature; to be old enough to be allowed to do something (vote, drink etc.)
Rapid transportation came of age when the first jets were built.

When my cousin came of age we had a big party for him to celebrate.

[TahlilGaran] English Idioms Dictionary ▲

of age
adj. phr.
1a. Old enough to be allowed to do or manage something.
Mary will be of driving age on her next birthday.
Antonym: OVER AGE, UNDER AGE.
1b. Old enough to vote; having the privileges of adulthood.
The age at which one is considered of age to vote, or of age to buy alcoholic drinks, or of age to be prosecuted as an adult, varies within the United States.
2. Fully developed; mature.
Education for the foreign born came of age when bilingual education was accepted as a necessary part of the public school system.

[TahlilGaran] English Idioms Dictionary ▲