Maybe it's a magical twist. There could be a prophecy that states Harry must take on the role early to save Hogwarts from a great threat. So, the wizarding world bends the rules for him.
In this fanfiction, it might be that there was a huge crisis at Hogwarts when Harry was 11. All the other potential candidates for headmaster were unavailable or incapacitated. Harry, with his already proven heroics, was seen as the only hope. The Ministry of Magic, in a desperate move, appoints him as headmaster. He then has to grow up really fast and learn how to manage the school while still being a child in many ways.
Chapter 814, "Under the Eaves, One Must Bow", is the clearest early answer because it turns Lin Manru's situation into a concrete plot event. The chapter gives readers a real answer by opening on Two little glutinous rice dumplings were lying by the coffee table...
A practical answer is Chapter 8, because it turns the idea into a concrete scene. The chapter opens with The resentment that had barely calmed among the crowd surged back two..., adds pressure through Why keep harping on the fact that he wasn't a decent man?, and ends on Other people with a.
Chapter 18 is the strongest chapter-level answer because it starts from In the corner, a girl in a black dress sat quietly, holding a glass of... and follows through on Hurry home, what is a young lady like you doing running to a place like....
Chapter 807, "If You Dare, Go Complain Now", is the clearest early answer because it turns Lin Manru's situation into a concrete plot event. The chapter turns that into something concrete by moving from Jiang Bufan had barely eaten dinner, having spent the whole day being...
Chapter 689, "Is the Soundproofing of This Private Room So Good?", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. It opens with Jiang Li found the restaurant Shang Shaobai had mentioned, and had Ji...
Look to Chapter 12 for the clearest answer. It begins with Qin?" Jiang Li called out twice, and only then did Mrs., keeps building through Qin, who was standing by the dining table, respond., and lands on The glass shattered on the floor with a crash, attracting the attention of....
Look to Chapter 33 for the clearest answer. It begins with Following the announcement, five artists respectively reposted the Weibo—..., keeps building through Instantly, "Shining Transformation Trip," already under intense scrutiny,..., and lands on Although Su Yinwan was considered half a.
Chapter 681, "She Actually Snatched It Away?!", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event. The chapter gives readers a real answer by opening on Shao Changqing was completely in awe of Jiang Li, and after his...
Chapter 866, "Why don't you die in his place?!", is the clearest early answer because it turns Jiang Meng's situation into a concrete plot event.
A practical answer is Chapter 23, because it turns the idea into a concrete scene. The chapter opens with Although Jiang Li exerted very little strength, Jiang Yan instantly felt..., adds pressure through So the moment the news broke that Jiang Li was entangling with Shang..., and ends on However.