Hypothesis I haven't seen mentioned yet: contraceptive pills affect women's tastes in men. Specifically, the ideal of male attractiveness for young women has shifted away from older, more rugged stereotypically masculine "successful" older man, to, well, Zac Efron.

By no means the only hypothesis, there are many good ones mentioned already, I just didn't see this one and it's plausible that it's at least a contributing factor.