Why do men choose younger women, and women choose older men?

Men and women choose their partners accordingly because they are trying to procreate. It relates to prehistoric times when men were trying to reproduce as much as they possibly could and women wanted to be stable figures who would nurture and grow their family for their mates. When men choose younger women, this is their most viable source of fertility and when women choose older men, it would be their most viable source create a stable home and provide security. So, ultimately it revolves around reproduction and population growth.