The Bible does not tell us the age of Mary and because God is a loving father, it wouldn't make sense for him to use a child to give birth.
The Bible states that she was a virgin, not a child.
Luke 1:26-31 In her sixth month the angel Ga′bri·el was sent forth from God to a city of Gal′i·lee named Naz′a·reth, 27 to a virgin promised in marriage to a man named Joseph of David’s house; and the name of the virgin was Mary. 28 And when he went in before her he said: “Good day, highly favored one, God is with you.” 29 But she was deeply disturbed at the saying and began to reason out what sort of greeting this might be. 30 So the angel said to her: “Have no fear, Mary, for you have found favor with God; 31 and, look! you will conceive in your womb and give birth to a son, and you are to call his name Jesus.
Matthew 1:22-25 All this actually came about for that to be fulfilled which was spoken by Jehovah through his prophet, saying: 23 “Look! The virgin will become pregnant and will give birth to a son, and they will call his name Im·man′u·el,” which means, when translated, “With Us Is God.” 24 Then Joseph woke up from his sleep and did as the angel of Jehovah had directed him, and he took his wife home. 25 But he had no intercourse with her until she gave birth to a son; and he called his name Jesus.
Is this reasonable? Surely it was not impossible for the Creator, who designed the human reproductive organs, to bring about the fertilization of an egg cell in the womb of Mary by supernatural means. Marvelously, God transferred the life-force and the personality pattern of his firstborn heavenly Son to the womb of Mary.