You make a good point. While I agree with the authors main thesis, I don't find his bullet points very persuasive, because they deal mostly with leisure activities. To decide which are more mature is largely subjective. As you pointed out, it's hard to make the case that one choice of leisure activity (watching SpongeBob) is more mature than another (watching Sex And The City).
Well said. After all, 50 or 100 years ago a predominant form of American entertainment was to watch grown men trying to smack a ball with a wooden stick.
IMO, a legal adult who is self-supporting, law-abiding, civic-minded, and respectful of others is a "grown-up", even if he reads Harry Potter books. A much more persuasive argument could be made by showing how fewer American men step forward to take care of the children they create, or how people are less responsible with handling their debts now vs. X years ago.
Of course, this has already been written about in hundreds of books and is not nearly as amusing as reading about Spongebob.