Donald Trump’s presidential campaign famously promises to “Make America Great Again.” As a rule, no one should think too hard about the meaning of bumper sticker political slogans. But ever since Trump rolled it out nine years ago, his slogan has quietly asked some seemingly unanswerable questions: When was America great before? And when did that greatness vanish, thus necessitating its recovery? The answer, it turns out, isn’t a year. It’s an age. And that age is roughly when you were between 11 and 16 years old—regardless of when you were born. Across generations, Americans seem to believe t…