The idea of kids marrying young isn't entirely accurate. They did marry much younger than today but 13 was still pretty young for the Western World for the past few hundred years or so. There is an excellent article here already on the myth but I'll paraphrase it. Basically, looking at marriage certificates/Church records in the 17th and 18th C in the US the average age to marry for women was anywhere from 20-22 based upon the location and time period. This was pretty consistent up until the 1990's when the average age of marriage jumped from ~21 in the 1970's-1980's to 24 in the 1990's and continues to go up.

So the change really was only in the past generation. :-)