This notion comes from history when young girls would be married off to much older men. In many cultures (both European and otherwise) the girls would be around 13 to 16 years old while there husbands could be in their thirties or older. This is because during the adolescent and young adulthood, it was necessary for boys to learn how to be men as characterised by their society - which could mean learning how to hunt, create and win wars and take care of a village or spending long years in school learning language, politics and philosophy as was the case in ancient Greece and Rome. Contrastingly, young girls were taught domestic duties such as cleaning, cooking, taking care of the household and children etc. That in addition to the fact that life expectancy during these times were relatively short due to lack of modern medice and that many women would die during childbirth (as well as the onset of menopause and ideal childbearing periods of that time) women were considered to have a limited period during which to have children. Thus men being older than their wives is really just a remnant of the patriarchal nature of society that has existed for many generations.