This article will answer two very common questions:
“How old was Mary (the mother of Jesus) when she became pregnant?” & “How old was Mary when she gave birth to our Lord Jesus, the Christ?”
Thousands of paintings, drawings, statues, and even “living manger scenes” portray Mary, Jesus’ mother, at the time of Jesus’ birth. However, most of them portray her as a young woman in her 20’s. To be sure, Mary was a remarkable woman. For one thing, she truly understood what it meant to sacrifice for the sake of doing God’s will, an example her son, Jesus, noted and would later follow. For example, apparently no one at that time, including she herself, was expecting the Messiah to be born of a virgin. This is why she asked the angel Gabriel how she was going to bear the Messiah when she was not married and sexually active (Luke 1:34), why Joseph decided to divorce her, and why, years later, the religious leaders still thought that Jesus was a bastard child, born of fornication (John 8:41 – KJV). She knew that accepting pregnancy from God would cause turmoil and pain to her and her loved ones, but she accepted God’s plan for her life, saying, “…Behold, I am the servant of the Lord; let it be to me according to your word…”(Luke 1:38 – ESV ).
One very remarkable thing about Mary is that she would almost certainly have been 12-14 years old when the angel Gabriel appeared to her. We know this because the common custom at that time was for girls to marry early, at that age. The Bible never gives Mary’s age when she got pregnant or gave birth to Jesus, and that is because when something happened that was common in the culture, nothing was said about it. We write the same way today, leaving out any details that everyone knows by custom and culture. If we read in the newspaper that a thief drove down the road for miles while being chased by police, we do not think, “Drove? What is ‘drove?’ I wonder how he got down the road all that distance?” We all know what “drove down the road” means, so we exclude the details when we write about it.
Similarly, if I tell someone, “I called Dan and talked with him,” it is clear from our culture that I called on a telephone, even though I do not say it. Many years ago I was teaching my kids the Bible, and I read them that Deborah the prophetess “called Barak” (Judg. 4:6 – KJV). My little daughter interrupted me and wanted to know Barak’s phone number. Her comment made sense given her understanding of custom and culture.
In ancient Israel, girls married in their teens, even early teens.  For Mary to be betrothed (engaged) but not yet formally married, yet old enough to have and nurse the Messiah, she would have been 12-14. The Zondervan Pictorial Encyclopedia of the Bible points out, “It appears that both boys and girls were married very young. Later [after the New Testament period] the rabbis fixed the minimum age for marriage at twelve for girls and thirteen for boys.” 
Some customs of biblical Palestine continued through the centuries, and after her trip to the Near East around 1910, Alma White commented on the age of marriage in Palestine, “A girl is usually married in her twelfth or thirteenth year, and sometimes as early as her tenth year.”  W. M. Thompson, a missionary in the Middle East for some 30 years, attests to the same thing.  James Neil points out that everyone married, because they felt obligated to fulfill God’s command to be fruitful and multiply, and that, “Girls are ‘given in marriage’ at eleven or twelve years of age, though this is not the limit. They are frequently married as young as nine….” 
Marrying young was the custom in many ancient cultures. Two cultures that had particular influence on the biblical world were the Greeks and Romans. Isomachus, a character in the Greek writer Xenophon’s Oeconomics, mentions that when he married his wife, she was not yet fifteen.  Roman girls also married very young, sometimes even before puberty. 
Women married early because it was they who bore the children who continued the family line and provided economic strength and physical protection for the family, and it was generally desirable to have many children. Furthermore, lots of children died young, and many women died in childbirth. We know from Matthew and Luke that Mary was a virgin, and had never been married before, so her marriage age would have been set by common custom. Also, as was common, Mary had a large family, with at least seven children. Her male children were Jesus, James, Joseph, Simon and Judas (Matt. 13:55), and she had at least two daughters, for Jesus had “sisters” as well as brothers (Matt. 13:55 and 56). 
Boys also married early, but somewhat later than girls. Jewish boys were expected to marry at sixteen or seventeen.  Similarly, in the Roman world, the age at which a Roman boy discarded the white toga with a red border, the symbol of youth, and wore the pure white toga of a Roman man and citizen, was generally between 14 and 17.  Just how old Joseph was when he married Mary is not as easily determined as Mary’s age, because although men customarily married in their mid to late teens, for a number of reasons they sometimes married later.
It is often assumed that Joseph married late in his life, because it is almost certain that he had died by the time Jesus started his ministry. We draw this conclusion from a number of biblical records in which Joseph was conspicuously absent, culturally speaking. One is when Jesus moved his headquarters to Capernaum his mother and brothers came, but not his father (John 2:12). Even more significant is that while on the Cross, Jesus instructed the Apostle John to take care of his mother and told Mary to treat John as a son, which would never have occurred had Joseph been alive (John 19:26 and 27). However, the fact that Joseph was dead by the time Jesus was 30 does not necessarily mean that Joseph was a much older man when he married Mary, because many people died young from accidents or disease.
If we are going to paint a picture of the birth of Christ, or cast a woman for a living nativity scene, there is some room for interpretation, but if we want to show Mary as she actually was, having a girl of twelve to fourteen as an actor or model would be the most proper thing to do.
 Scripture quotations marked (ESV) are taken from The Holy Bible, English Standard Version™ © 2001 by Crossway Bibles, a division of Good News Publishers. All rights reserved.
 Philip King and Lawrence Stager, Life in Biblical Israel, (Westminster John Knox Press, Louisville, 2001), p. 37.
 Merrill C. Tenney, editor, The Zondervan Pictorial Encyclopedia of the Bible (Regency Reference Library, Grand Rapids, MI, 1976), Vol. 4, “Marriage,” p. 96.
 Alma White, Jerusalem, Egypt, Palestine, Syria, (Pillar of Fire, Zarephath, New Jersey, 1936), p. 83, 173.
 W. M. Thompson, The Land and the Book, (Baker Book House, Grand Rapids, MI, 1880, reprinted 1973), p. 293.
 James Neil, Everyday Life in the Holy Land, (Society for Promoting Christian Knowledge, London, 1913), p. 223.
 Robert Flaceliere, Love in Ancient Greece, (Crown Publishers, Inc., New York, 1962), p. 110; Mary Johnston, Roman Life, (Scott, Foresman and Company, Chicago, 1957), p. 145.
 Robert Flaceliere, Love in Ancient Greece, (Crown Publishers, Inc., New York, 1962), p. 115.
 It is sometimes said that these brothers and sisters were not Mary’s children, but that argument is not based on good exegesis or the natural reading of the text. The people of Nazareth, who made the statement, were not making a comment about Jesus’ relatives, but about his family.
 Alfred Edersheim, Sketches of Jewish Social Life, (William B. Eerdmans Publishing Company, Grand Rapids, MI, reprinted 1979), p. 147.
 Mary Johnston, Roman Life, (Scott, Foresman and Company, Chicago, 1957), p. 146.