There is no set age for marriage throughout history. Economy, resources, and the meaning of marriage effect the age at which people formed unions. First, in ancient history, females in many cultures did not exist as individuals on their own. In fact, in ancient Roman families, girls were not even given names. Throughout much of the time of the Roman Empire, girls were know officially by the feminine form of their family name, and when there were multiple sisters, they would be designated by a number after the family name. So, overall, girls were more like property that was transferred from one family to another to seal transfers of real property and to create connections between families. As a result, the age of marriage could be quite young. Among wealthier families, we are talking mid-teens. Women would keep their families name when married, but they would also take on a name that identified them in relationship to their husbands. It wasn't until the later part of the Roman Empire that some women started getting their own given names. All of this ignores the poor and the middle classes. We don't know too much about their marriage patterns.

Consider the Middle Ages, where again, marriage was for the rich and wealthy. We do know that only occasionally in England would peasants marry before priest. And in fact, marriage was more of a legal contract more than a social or even religious contract. The poor or middle classes would normally simply stand at the porch of a church and declare themselves married. While marriage among the aristocracy still saw girls being married off in the early to mid-teens, there were economic considerations of the middle and the lower classes to consider. Could a man afford to take a wife who might not be skilled enough to help bring in income? Marriage itself was a way of ensuring economic prosperity. By marrying, a man would avail himself of services for which he would have to pay.

We start getting a clearer picture of when the average people are marrying by about the early 1300s, as there are greater attempts to keep track of the populations of various countries, through church records. English records show that in the 1300s and 1400sh century the age of first marriage for most English women was between 18 and 22. By the 1600s, the age of first marriage had advanced to an average of 25. As it became increasingly more expensive to get set up in life and raise children, marriage was postponed.

When the colonies began to take hold in America, the age of first marriage here dropped dramatically, with girls getting married around 18. And yes, Europeans were shocked at how young girls in the Americas were getting married in the early 1600s. But as the colonies grew, marriage ages fell off, except near the frontiers, where there was an economic imperative to marry and a decreased cost in starting your children off in life.

By the early 1800s, the average age of marriage for women in Massachusetts had risen back up toward the British norm of the 1600s (mid-20s). But out in Ohio, women were marrying significantly younger (late teens to early 20s). Throughout the USA, marriage ages in the 19th century dipped and surged by localities and the general prosperity of a time.

I think you would find this article on the economics of marriage interesting; http://onlinelibrary.wiley.com/doi/10.1111/j.1468-0289.2009.00483.x/full

A popular book about the history of marriage is Stephanie Coontz, "Marriage: A History." And while it is very readable, I get frustrated by the lack of citations and documentation, so if you choose to read it, please note that it is a bit loosey-goosey.