The image comes from a blog post by Robert Brewer, a motivational speaker. Brewer wrote the blog post years ago, but later removed said image after he received complaints and discovered that the research he had used to create the image was bad. He even wrote a follow up post in 2015 about the lessons he learned from the experience. I have to tip my hat to him as it's not easy to admit when you're wrong, especially in a situation like this where you didn't intentionally set out to spread misinformation.
However the diagram did remind me of things I'd heard from various people throughout the years. I remember working at a video store and recommending books based on customers' video rentals, only to be told that they hadn't really read anything since school (college or high school) beyond magazines or the occasional textbook. I've also heard people say that they haven't been to a bookstore in years, although I wasn't always able to ascertain the context and whether they meant that they hadn't read anything since then or that they got their books from somewhere else. I'd had many situations where it looked like it could be a case of either.
I think that part of the explanation for reluctant adult readers is because of the lack of variety of reading materials in high school. Not every student was encouraged to read at home and/or have access to reading materials. Their parents wouldn't necessarily discourage it, but they wouldn't always have access to reading materials and their parents wouldn't always encourage them to read things that interested them. I've heard multiple people say that they found reading in high school to be dull and boring, as they just didn't connect with the characters in the classic works you typically find in North American classrooms. This was possibly because of the subject matter, but also possibly because teachers couldn't go into the more salacious subject matter out of a fear of losing their job. It's likely why none of my history classes during my public education discussed the fact that Benjamin Franklin was a huge ladies' man or that he played pranks like his life depended on it. Even the whole Thomas Jefferson and Sally Hemmings thing was covered quickly and swept to the side, lest a parent complain about their child learning about Presidential sexual shenanigans. (And potential rape depending on what materials you read about Jefferson and Hemmings.)
This is why it's so important for classrooms to allow their students to explore literature on their own before starting in on the classics. A student will best appreciate something if they appreciate reading to begin with and they'll also be more likely to continue reading as adults. Using a variety of reading materials can also help, as more reluctant readers might be more open to reading a story in graphic novel format. This is a format that has long been maligned as "frivolous", however any comic book or graphic novel fan can easily attest to the fact that graphic novels can provide a rich and insightful look into various different cultures and lifestyles. It can also help provide training of a sort, as I introduced a friend's child to manga. The traditional right to left reading format helped the child, who suffered from mild dyslexia, form an attachment to reading. Once she got hooked on reading graphic novels, she branched out into YA novels and the like. Now a beautiful young woman, she's a voracious reader of anything she can get her hands on.
I'm glad to say that many schools are trying to allow their students the freedom to read whatever they want rather than pointing them only at specific books. More schools (and parents!) need to do this in order to keep things going. Keeping the chipper spirit going, there are also more adults reading. Whatever you might think of stuff like Fifty Shades and Twilight, it did get a lot of adults to start reading. I'm also happy to say that there are far more initiatives aimed at children and teens, where the goal is to provide them with free reading materials and inspire a love of reading. Lots of local groups tend to donate books to schools, hospitals, or other locations where children can go to pick up books - however unfortunately the areas that tend to need this the most are usually the hardest to reach.
I suppose the only thing we can do is to continue reading and try, when possible, to encourage others to pick up a book. I remain adamant in my belief that there's a book out there for everyone.