I don't get this, she is married to Joseph? But if we assume she is a good Jewish girl and obeys the law, elsewhere in the bible it describes consummation of the marriage and sheets being shown about as proof as virginity. Yet we are told she is a virgin, so the marriage to Joseph couldn't have been consummated. Which would mean that she wasn't properly married and Jesus was born out of wedlock? Unless it was custom not to consummate a marriage if the bride is this young? It raises all sorts of questions when you analyze it.
Based on cultural norms and average life spans of that time period it is assumed she was between 13-16 years of age. On average people lived to ages 40-50. Women married at ages we considered young and had children at the ages the were most fertile and most likely to have healthy live births because infant mortality was high in those days and the older a woman was the less likely she was to have a healthy living child and her own chances of dying in childbirth increased. In her day 14-15 would have been a perfectly normal age for a young woman to be married and starting a family. Men, on the otherhand, waited until theybestablished themselves in life so they could support a family so men tended not to marry until much older so it was the norm for young women of 14-15 to marry men 10-20 years older than them or more so its likely Joseph was in his late twenties or his thirties when he married Mary which was basically middle aged which could be why he died before Jesus began his ministry.