Men have a strong biological urge to mate with that that represents vitality, health, and fertility; therefore, once they sexually mature, they have a tendency to be attracted to females exhibiting those characteristics. For a period, I’d say until early 20’s, available partners are equally young and green, so most people date within their age range. However, as time goes on, the girls mature faster, creating an emotional disparity and the guys start looking at younger girls as potential mates.
Not all men like seriously younger women, but I think many men’s heads are turned by a beautiful, fresh-faced, fertile woman. I also think the desire for younger women is proportionate to men getting older (loss of vitality in self, etc.). You start noticing the upward trend of men dating younger women as the men become older. Much more so than the other way around (older woman, younger man)... it all goes back to that biological imperative, that men are usually attracted to that which radiates health and fertility.