Why is there a double standard on older/younger men/women dating and hook-ups?

Because women are often reluctant to date men significantly younger than them. The fact is most women would prefer to date a man who is slightly older, if given a choice. So, when a woman is dating a man that much younger, it is often in spite of his youth, not because of it. Whereas, men are happy and often prefer to date someone significantly younger, for no other reason than their youth. So, basically women who date younger men are often less shallow than men who date younger women.