There was a world where everyone was expected to be the same height at the same age. People who were shorter than that height were considered behind in assorted different ways (their body proportions determined which way), and people taller than that height were considered gifted but also weird and possibly shunned as well.
In that world was born a boy who tended to be about a foot shorter than he was expected to be. The boy learned early on to walk on tall stilts so that he would be the same height as everyone else, sometimes taller. He learned to walk on those stilts without touching them with his hands. When he fell, it confused people, but he was always able to get back up on the stilts. When he was on the stilts, he was treated as just the same as everyone else, or as advanced, depending on how high the stilts were on any given day.
As he got older, he contracted a balance problem. It was not a particularly severe balance problem. In a person who could walk without stilts it would make little difference at all. But to this boy, it made all the difference in the world: He could no longer walk on stilts. The best he could manage were fairly short platform shoes.
Everyone was scrambling to figure out what had happened to him. After all, he had been “normal” or “ahead” (if walking a little funny and doing things that people couldn’t understand when he had to get off his stilts), and suddenly he was walking on the ground, his normal height.
Meanwhile he became adept at a whole lot of things he had never been adept at before: Running, climbing, jumping, all kinds of things that could be done without stilts. His minor balance problem did not get in the way of these things, but it did get in the way of everything he’d been able to do because of the stilts.
People assumed he had shrunk and tested him for all kinds of conditions that can cause shrinkage, even though he hadn’t shrunk at all, he had merely lost the ability to use his stilts. People questioned his ability to be described as the person of the height he actually was, because he had for so long worn stilts to make himself taller. Some believed that he was too lazy to use stilts, even though every time he used stilts he fell on his head. Others assumed he had never used stilts in the first place and were astonished at all the things he had learned to do while able to stand at the same height or taller as everyone else (this society was inaccessible to people shorter than the designated height, so shorter people did not have certain advantages), thinking he must have benefited from recent techniques for teaching shorter people (when he really hadn’t, although he now made use of some of them since he couldn’t use his stilts anymore).
But all he was was a boy who was shorter than average and had made himself stilts to walk on, and eventually become unable to use the stilts anymore. He had not shrunk, he had not in essence changed, he had just lost a particular rather minor ability, and for that matter gained several others in the process. The only reason it seemed like a major change was because his society valued apparent height over nearly all else, and it overshadowed their ability to perceive what had really happened to him.