Mary must have been very old. God is the Alpha and the Omega.
But when he chose to become flesh and dwell among us he needed a body, and he got that from his mother whom he created:
Gal 4:4 But when the fulness of the time was come, God sent his Son, made of a woman.
And as the Holy Spirit proclaims in Luke through Elizabeth, this woman is the Mother of God:
Lk 1: 41-43 "And it came to pass, that when Elizabeth heard the salutation of Mary, the infant leaped in her womb. And Elizabeth was filled with the Holy Ghost: And she cried out with a loud voice, and said: Blessed art thou among women, and blessed is the fruit of thy womb. And whence is this to me, that the mother of my Lord should come to me?"
When the Word became Flesh, he was borne by a Mother. That earned her the title of Theotokos.