be of age

To be at the age of majority—that is, old enough by law to vote, marry, or sign legal agreements without one's parents. Typically refers to age 18, the age of majority in the United States. Just because you're of age doesn't mean you're suddenly versed in the ways of the world—you're still just a kid. Our parents don't approve of our marriage, but we're both of age now, so they can't stop us. This money will stay under the protection of her legal guardians she is of age.