1.Prepositional phraseOld enough to be considered an adult (in law, or by society generally).
"He's of age now: he can make his own decisions."
2.Prepositional phraseHaving existed for a certain period of time; used in referring to a person's age.
"He has a son who is six years of age.; One must be 18 years of age to be an eligible voter in most countries.; Once one is of age 16, one's horizons broaden; once one is of age 21 one's horizons are limitless."