In Mark 5:41-42, a 12 year old is referred to as a little girl. Mary was always called a woman in the Bible. So there is a strong possibility that she was much older than 12. Likewise, in Luke 1:42, Mary is referred to as a woman - "Blessed are you among women" - even before Jesus is born.
I have more evidence to support the claim that Mary was older than 20 when she gave birth:
God in the Bible called all those below the age of 20 as children
“Surely none of the men who came up from Egypt, from twenty years old and above, shall see the land of which I swore to Abraham, Isaac, and Jacob, because they have not wholly followed Me.”
“Moreover your little ones and your children, who you say will be victims, who today have no knowledge of good and evil, they shall go in there; to them I will give it, and they shall possess it.”
Mary was always called a woman in the Bible. In Luke 1:42, Mary is referred to as a woman - "Blessed are you among women" - even before Jesus is born. This proves that Mary was older than 20 when she gave birth.
Proof from Bible manuscripts that Rebecca was 20 when she married Issac.
According to the Book of Jubilees (xxxi. 8-11, 48), Jacob, when he arrived home, found his mother alive; and she afterward accompanied him to Beth-el to accomplish his vow (comp. Gen. xxviii. 19-20). She died at the age of 155, five years before Isaac's death (Jubilees, xxxv. 1, 41), this determining that her age when she married was twenty years.
The Book of Jubilees is found in the manuscripts of the Dead Sea scrolls and is part of the canonical Bible of the Ethiopian Orthodox Church.