(tentative title) A Century At Age 10 - The Saga of the Prolonged Youth
This will be a series of novels too. In fact, I cannot think of one planned novel that will only be a single standalone book.
A 10-year-old boy (name undecided, but will be a Korean boy) celebrates his 10th birthday, and hikes out to the woods that afternoon to give a little time to himself. (As it goes without saying, he receives loads of attention from his friends & family that day.) A supernatural entity (whose physical features are hard to describe) tells him that he is to stay 10 years old for the next 90 years, then age one year every decade after the year 2000. He is the subject of an experiment to determine how refined and learned children can possibly be.
He thought he had hallucinated, but by the time he's 12 1/2, he notices that his friends and family are physically maturing. Others start to notice that he hasn't. He buys himself a little bit of time by saying that "Some kids bloom later than others. It'll come eventually."
But now knowing that his 10th birthday encounter was real, he secretly prepares to run away in order to protect his family. He spends his life drifting to different homes, orphanages, youth hostels, and other available places to live.
He still learns new things, matures socially and in other ways too, but the fact that he remains a kid when he should've grown up already keeps troubling him at the same time. Every time he goes to a new town & school, he makes up an alias and that he's 9, so that he buys himself a few years at that one location. He opts for a placement test and gets "skipped up" to middle school (and eventually high school later in his travels.)
Many years pass and he tries adapting to the changing society as much as he can. Then the story reaches the Present Day, and his body has matured, albeit a whole lot more slowly as aforementioned. By this time, he knows a wide range of trades, makes friends easily, takes college courses, and even invents new tools and toys.
However, some people have been following him, trying to figure out what his mystery is behind his prolonged childish state. Sometimes, he does tell people that he's never aged in many decades, but only when he knows they'll think he's making up a tall tale.