That is the big question - Mary did you know?
To me Mary was created to be the Mother of God. At her age, did she know? Fourteen years old when our Saviour was born of her.
She is a model of what it means to have faith and trust God. The angel Gabriel appeared to her and asked her to be the Mother of God. She was old enough to know that a male was needed for that to happen. When the angel told her that the Holy Spirit would give her the child, she didn't question it - she just made her choice to trust in the Glory of the Lord. Then the "yes" was given. Her consent to let God work through her, gave us the greatest Gift that we could ever be given.
She knew that if people found out that she was an unwed mother, they would probably stone her to death. She had complete faith in God's plan.
She knew she was chosen to nurture, care for and be Mother to the Son of God. She knew that a Savior was coming and she knew that she was to be His Mother.
Oh Mary, ever virgin, thank you for saying yes to God and having faith and trust to let his will be carried out through you. Amen.