As recently as the 1600s, being unmarried by 14 was akin to 'death' for girls. Mary could easily have been around 10 at the time of conception/betrothal. There is little verifiable information on this, however, other than what is statistically available but quite often girls as young as 8 or 9 have been known to get married in those days.
It had a lot to do with high infant mortality rates.
Men, on the other hand, didn't often marry until their mid-twenties, by which time they would have learned a trade with which they could support a family.
EDIT: Two additional points that I feel need to be added.
1. Jesus did not NEED to be born of a virgin, but such a birth would be seen as a sign. The concept of original sin (or the avoidance of such) tends to work out like a snake eating its own tail and fails when placed under logical scrutiny.
2. It is very unlikely that Mary remained a virgin after the birth of Jesus, and it is suggested that Jesus had other siblings, one of which is mentioned in the Bible. Joseph, by right, was entitled to fathering children and would probably have taken Mary to bed just a few months after the birth of Jesus.