Reading decline happens during the big transition all middle-grade readers go through: moving from short illustrated chapter books to fully fledged novels. | Ryan McVay/Getty Images Something happens to kids when they’re about 9 years old. Before then, they’re voracious little bookworms, with most of them saying they read for fun at least five days a week. After their ninth birthday, only 35 percent still do. Experts call this phenomenon “the …