Because men can sire children at later ages than women can have children. The issue when these faerie tales were conceived, passed down orally, and then written was all about fertility AND for women, that they be beautiful. A 36 year old woman was not as fertile as a teenager/20 something. Even though there are handsome princes in the tales, it was more important in real life that men be rich and able to take care of a woman. Hence, a prince rather than some guy working in the field--but only if the woman were good-looking. If she were ugly, she would not be sought after. Cinderella was nothing until she was cleaned up. Snow White was beautiful and so was Rapunzel and Sleeping Beauty. Think of Cinderella's ugly stepsisters--they couldn't have been much older than she was, but no one was interested in them. Yup, they were portrayed as mean, but had they been gorgeous, their meanness would have meant at much. Also, beauty was largely equated with goodness; Sleeping Beauty's wicked but beautiful stepmother is an exception.

But that has little to with why in the modern age, older men are cast as the prince. Unless the king were really, really old, princes would be young. However, the woman still needs to be very young per the old tales as women are judged by their youth and looks in any era.