I think it has to do with biology mostly. The female becomes less fertile as she ages whereas the male is able to conceive throughout his life, assuming no other problems are present. In the olden days men frequently had 3 or more wives because it was common for women to die in childbirth. Both of my grandfathers had 3 wives each. Also, another reason could be that isolation/geography and travel were limited and the pool of available spouses was frequently limited by economics as well. Who could afford a wife was usually the question, not who was the right age. The Hispanic culture especially in the American southwest is very traditional in this respect, but is becoming less so. An interesting topic.