It was normal in the Near East in the First Century for girls to marry at about 15, Christian girls usually a little later but not much, and menarche was usually about 12-14 (compared to 11-13 in the USA nowadays). This was to prevent their becoming pregnant by someone who wouldn’t marry them, and sometimes to ensure the parents had a say in the choice of spouse. Boys married a little later, because their preparation for marriage was longer, since they had to be able to pay for the bride, wedding and often a house to bring her to by the time of the wedding. Joseph was apparently a widower. Because there was not to be sex in their marriage, the normal reluctance to give a girl to such an old man wasn’t there. He married her to keep her safe. A husband then had to defend his wife and children physically or pay someone to do so out of his earnings.
Girls would have married even earlier except for a few problems. One, girls married before they want to be often run. Then they are endangered. Two, early pregnancy and very early sexual activity are harmful and can shorten a girl’s lifespan a lot. Three, a man in those times wouldn’t generally want a wife who didn’t have a lot of skill at textile work and childrearing, and usually other work as well. It took some time to develop such skills. But a girl who wasn’t married by age 18 sometimes ran off with a man unacceptable to her parents. And sometimes she turned up pregnant with no fiance at all.
Under the biological conditions of modern America, if a woman wanted the greatest possible number of children, statistically her best bet would be to marry at 15 and give birth the first time at 16. Earlier childbearing can damage the reproductive system or even kill the young mother, and isn’t as likely to produce children who survive to adulthood, but waiting longer cuts off more reproductive years. It would be a tradeoff. But modern American girls who want babies don’t usually have the option of geting married. In the 1950’s and 1960’s more young girls did get married, some after conceiving their first child, some before. But imagine a world where the ruling people are the ancient Romans, and you live in a colony of Rome. People are poor and oppressed. To get ahead means cozying up to the pagans, which would make you a pariah among your people for life. The life of a simple laborer is almost impossible even for a man. For a woman it’s much tougher. The only disability insurance there is is children. You must have kids – but to have them on your own is both wrong and an economic catastrophe on the farm. If your parents are too old to help you and your kids are too young to help you and you get hurt, you may not eat. It wasn’t a preference but a struggle to survive that made people marry young.
