"One of the common claims is that Mary was probably only a little girl of 12-14 years old. The reason given for this is that it was "common custom" in those days to marry girls off that young. It is also suggested by some that Joseph must have been much older than she was because he appears to have died early. There is no mention of him after Jesus was 12 years old. This is an assumption and not necessarily based on the custom of the time.]
I don't think we can determine Joseph's age, but I think it is not likely that Mary was as young as some people represent her.
First of all, while I realize that Joseph and Mary were already espoused - but not officially married - I find it interesting that her parents don't even get an honorable mention here. We're not introduced to her as "Mary the daughter of _________". Added to that, Mary gets a lot more "press" than even Joseph does, especially in this passage. If Mary was just a little 12 to 14 year old girl betrothed to marry an older man, that seems a little bit odd. In such a case her parents would have still held a large place in her life because Joseph hadn't taken her yet, Matthew 1:18-20.
Some people might like to use this to show that a girl/woman belongs to her husband-to-be immediately upon betrothal/espousal, but that doesn't exactly make sense. When a young lady is promised in marriage, but not yet taken, and is still in her father's home, she must be in a situation of being answerable to both men for a time of transition. Assuming Mary was only 12-13 years old, she would surely have been living with parents or guardians still. One would expect that either Joseph or Mary's father would have been the main character being dealt with by God, and yet neither one is. That is a very interesting point and makes it hard for me to believe that she was that young. The fact that God deals directly with her and there is no mention of her father makes it seem likely that she was a good bit older than that, especially since in v. 39 she apparently travels by herself to see her cousin, Elisabeth.
Another reason I don't believe Mary was such a young girl is because in Mark chapter 5 we meet Jairus who came to Jesus pleading with Him to heal his daughter. In Mark 5:23 we read, And besought him greatly, saying, My little daughter lieth at the point of death: I pray thee, come and lay thy hands on her, that she may be healed; and she shall live. Jesus went to the house and raised the girl from the dead, and in vs. 41-42 the scripture says, And he took the damsel by the hand, and said unto her, Talitha cumi; which is, being interpreted, Damsel, I say unto thee, arise. And straightway the damsel arose, and walked; for she was of the age of twelve years. And they were astonished with a great astonishment. First, it is just my observation that a man who expected to marry his daughter off in a year or less just might not be calling her his "little daughter" at age 12. I realize that emotions could be blamed for his use of this expression, but somehow it just doesn't ring true. But, more than that, the word "Talitha" is thought to be a diminutive word meaning "little girl", as opposed to a young woman. We might infer from that that Jesus also did not consider her a young woman of marriageable age.
Which leads to another point which I think is even more important. Just because something is the "common custom" of a given era, it does not follow that all those who love God and follow His word will be doing that thing. Imagine if this earth went on another 2000 years and much of our present history was lost along the way. Some "scholar" who was supposedly "in the know" might portray us all as having tattoos because "it was the common custom" of that day. And, yet, would that be true? No! Certainly not. Some of us don't have tattoos for Biblical reasons, in fact. Mary and Joseph both were obviously people who loved the Lord and followed His commandments. It is more likely that Mary was raised by parents who loved God than otherwise. They would not follow the customs of the day just because that is what everyone else was doing - even if the "good, godly" folks at the synagogue thought it was fine. There were a lot of things the "good, godly" people of their day were doing that Jesus later rebuked!
Furthermore, we know that the early Christians in the Roman Empire did not typically practice early marriage for their young girls. In his book, The Book that Made Your World, Vishal Mangalwadi writes (p. 284), "Christians also expressed their respect for women by raising the age of marriage. Roman law established twelve as the minimum age at which girls could marry. But the law was nothing more than a recommendation...and was routinely ignored. The best available studies show that in the Roman Empire the pagans' daughters were three time more likely than Christians to marry before they were thirteen. By age eleven, 10 percent were wed. Nearly half (44 percent) of the pagan girls were married off by the time they were fourteen, compared to 20 percent of the Christians. In contrast, nearly half (48 percent) of the Christian females did not marry before they were eighteen."
My point is that the people Mary was associated with - Zacharias, Elisabeth and Joseph - were all godly people who loved the Lord. Mary herself was obviously a godly woman. It is possible that Joseph and Mary had been betrothed at an early age, but he had not taken her yet, and based on some of the other things I've already mentioned, it seems more than likely that she was older than the legendary 12-13 years at the time this account opens.
My last reason for not believing that Mary was a little 12-13 year old girl is that it is not physically safe for a girl that young to have children. If no one else was watching out for Mary's safety you can count on the fact that God was! My loving, kind heavenly Father would not impregnate a girl that young and physically immature. He just wouldn't do it. He is not that kind of God. That kind of behavior is found in gods in other religions, but not in God Almighty, the Faithful and True. Of Him it is said, He shall feed his flock like a shepherd: he shall gather the lambs with his arm, and carry them in his bosom, and shall gently lead those that are with young. Isaiah 40:11 If there is no other reason to know that Mary was not that young it is this. The God we know and trust simply wouldn't do that to one of His daughters. (Yes, men might do this; men who professed to fear God might do this; but remember that in this case God was the one calling the shots, fulfilling the prophecy, and ordering every event. He was fully in charge in a way we don't always see when He's dealing with sinful men.)....
Please stop calling mary a little girl. She was not a little girl. STOP ACCUSING GOD!