Age is an attribute that means "time of existence". A person's age is the number of years since that person was born. The age of an artwork or a work of literature is the number of years since that work was created (not necessarily, published).
For people, age is often used as an approximate indicator of development and maturity, especially in childhood and adolescence. Legally, the age of majority is the age when a person acquires all the rights and responsibilities of being an adult and is no longer considered a minor.