In those days, 14 yo was a good age for a woman to marry as people did not have long life expectancies like they had had earlier in their history.
Because God knows the hearts and minds of all of us, and His people are preordained , He knew before Creation who would be the vessel to carry the royal child and heir. He knew her bloodline and ancestry, for it had to be someone who could trace their genes right back to Abram and the child of promise, Isaac,, to bring to fruition, the complete Mosaic Covenant, with 42 generations between Isaac and the Christ child; to fulfill Old Testament prophecies and to bring to completion The Torah, or the law of Moses as it was otherwise known as.
Luke 1:46-55 is the canticle of Mary... her song of being blessed.. It is also known as The Magnificat, for she knew she was blessed above all women, to be the chosen one of God, was a great honour....
People had a much closer relationship with God in those times , for He and His laws impacted every area of their lives and it had been known for many generations that a jewish girl from the Tribe of Judah would be the one to carry The Messiah..
To complete your knowledge of this you should read the whole Gospel of Luke, for his book writes in detail of the heavenly hosts that paid homage to Jesus Christ , as He lay in the manger...
The Gospel of Luke/ New Testament/ The Holy Bible..