A few decades ago, before TV commercials became obsessively concerned with prostate problems, Jack Kerouac wrote a book called “On the Road.” It was greeted rapturously by many as a burst of rollicking, joyous American energy. People quoted the famous lines: “The only people for me are the mad ones, the ones who are mad to live, mad to talk, mad to be saved, desirous of everything at the same time, the ones who never yawn.”

In the Times review that launched the book, Gilbert Millstein raved that “On the Road” was a frenzied search for affirmation, a book that rejected the ennui, pessimism and cynicism of the Lost Generation. The heroes of the book savored everything, enjoyed everything, took pleasure in everything.

But, of course, all this was before the great geriatric pall settled over the world, before it became illegal to be cheerful.

“On the Road” turned 50 last month, and over the past few weeks a line of critics have taken another look at the book, and this time their descriptions of it, whether they like it or not, are very different.