Mary was young—probably just a teenager—when the Angel spoke to her, upsetting her simple plans, in order to make her part of God's great project in Jesus Christ.
She remained young even afterwards, when, despite the passing of the years, she became a disciple of her Son with the enthusiasm of the young, and she followed him to the Cross with the courage that only young people have.
She remains forever young, even now that we contemplate her assumed into Heaven, because holiness keeps us eternally young, and it is the true "elixir of youth" that we need so badly. It is renewed youth that the resurrection of the Lord has brought us.
The Holy Father's video message for the International Marian Vigil with young people at the new Shrine of St Gabriel of Our Lady of Sorrows (Teramo, Italy), 12 May 2018