Prisoner of Azkaban, the third Harry Potter book, is also the third I’ve read to my 7-year old. The rule has been “one a year”, mostly because I know the series ramps up the level of darkness and anxiety, but also because once we hit the pagecount explosion of Book 4 I’m keener to pass the job on to him.

There’s some indication, I think, that this is how Rowling expects things to go. The first two books are really exceptionally good for reading aloud – clear storytelling, distinctive character voices, a steady flow of new ideas and exciting incidents, and evocative but simple description.

By HP3, that’s starting to change. The narrative is getting more sophisticated not just emotionally but technically. Crowd scenes are rendered with snatches of unattributed dialogue. Lists of books include parenthetical excerpts. These are ultra standard techniques, and a child reader gets the point of them very quickly. A reader aloud has more trouble making them work. They simply aren’t meant to be bedtime stories.

There’s also a lot more worldbuilding going on. The Potter books mostly start with a relative lull, then spend a lot of time on the books’ stock antagonisms (vs Dudley, vs Malfoy, vs Snape) while introducing new characters and quietly moving plot elements around in the background. There’s enough drama in the running enmities to make each night’s reading exciting even before the layered payoffs start hitting. But in HP3 not only are there even more spells, artefacts and magical concepts to deal with, there are huge chunks of backstory exposition, which have to be delivered in ever longer talking heads sequences.

Thankfully, these are pretty exciting bits of exposition, all about betrayal and murder. But the introduction of Sirius Black – the living link to Harry’s parents and past – crosses a rubicon. The B story he brings with him – the lives of the previous generation of Hogwarts’ students – never fades out: it gets more and more important (and hence the expository scenes get more crucial) across all the rest of the novels.

So this is a pivot point in Harry Potter. The books start as a kids’ series, and end it as something closer to a Young Adult one. A lot of older readers – me included – are reading it alongside their kids and like the kids’ end more, or at least think JK Rowling is better at handling it. And so Prisoner of Azkaban is a favourite, as it’s Rowling pushing the magical school story with its domino-topple plot as far as it can go. But if you look at 20something Harry Potter fans on Tumblr, they react to the sturm und drang of later books more – which get more and more influenced by that previous-generation B-plot, all about the consequences of teenage friendships and crushes, which is a much more YA set of ideas.

If the series had started out with the Young Adult mentality – i.e. if it had always been meant to be read by bright 13-14 year olds, not 10-year olds – maybe it would have gone for an Alan Garner type narrative (or one like Stephen King’s IT): two generations of storytelling in parallel, so you could trace the resonances between them and see how mistakes and decisions influenced each other. But that’s not how the series was set up – it’s Harry’s story, with the B-plot of 1970s Hogwarts existing as eggs laid inside it, slowly hatching.

The main consequence of this, it seems to me, is that the role of the adults in the books shifts dramatically. HP3 is the first book in which intra-adult dislike is a plot motor (as opposed to a joke – everyone thinking Lockhart is a dick, for instance). Snape hating Lupin is important, and requires kid readers to have a way of grasping and relating to adults’ interior lives and motives, which they won’t really have until a certain age. In the first and second books, the adults basically fill standard adventure fiction roles – their point is to be incomprehending enemies or wise mentors, while the kids carry the adventure. HP3 changes all that – the climactic scenes in the Shrieking Shack are all about four grown men dealing with that they did or didn’t do when young, while the child protagonists of the books are mainly onlookers.

I say “grown men”, but that’s the problem. Younger kid readers basically don’t have much of a way of grasping adult interiority – which isn’t to say kidlit can’t be sharply observational about adult actions and justifications, of course. Teenage fiction, on the other hand, is or should be excellent at capturing teenage motivation and interior lives. So that’s what the prior-generation characters – the motor of the series’ shift towards young-adult status – get defined by. What Snape, Lupin, Black et al. did at school and just after defines their entire role in the books and their emotional motivations: an unresolved teenage crush isn’t just character-defining, it’s the twist the entire seven-book series ends up resting on. By the end, Harry Potter is a book about teenage teenagers and adult teenagers and dead teenagers, which is its power and appeal and why I am stopping reading aloud after this third book, where the slow roll towards that endpoint begins.