It doesn't seem to me like it is a direct choice of adults to consume juvenile media, rather all media has adjusted to the curse of the marginal user and prefers to market garbage that can be sold to anyone.
That doesn't apply to books though. As books aren't advertised the same way as any other media. Most books are advertised just through reviews and word of mouth. People decide to read juvenile media because their juvenile friends recommended it to them.
That didn't apply to books, plans for movies and the multi media franchise are faster now, discussions on day time TV might be manipulated with largely false controversy to imply parents should read all this stuff, etc.
That being said, I don't see much connection between the G/PG blockbuster related adult readers and the coming-of-age book auditors.