A plot bunny looking for input

[The Wanderer]

(Argh. Nested quoting on these forums is a royal pain to make come out well.)

Presumably because Peter forced the agreement somehow

Peter and the other Lost Boys, yes - and the "how" is simple: they simply refused to agree to Harry's going unless he would be coming back. (Not that they necessarily would have stopped him if he wanted to go anyway, but do you really think any Lost Boy would be willing to leave Neverland if Peter told him not to?)

Although it would be interesting to see the wizards decide they don't need to abide by the agreement because they figure a boy in Neverland can't do anything to them if they abrogate it, only to find out they're very wrong...

Well, if I do write this, that would be unlikely to happen - largely because I neither like nor subscribe to the notion of an overcontrolling, my-way-or-nothing manipulative Dumbledore, and with him taking the honestly benevolent approach the odds that the others would actually go ahead with that reaction are fairly low. It's certainly an alternate angle on the situation, though.

he really shouldn't be older than about six

Physically. Chronologically, though... but then you note that already in your bunny text.

True - although note that chronological (perhaps better called "experiential"?) age specifically does not correlate with maturity, in this context.

I am so anticipating the first time he crows in the Great Hall.

I hadn't even thought of that. Thanks; that's the seed of a scene which can be developed...

Would he even be allowed to play for a House team, given his physical age?

Would he be disqualified for not needing a broom?

Hmm. If he is in fact interested in playing Quidditch, perhaps he would want it badly enough to be willing to promise to not fly in a game apart from the broom unless he fell off (and agree to be disqualified or at least penalized if he broke that promise), and that could be a hook by which he could begin to learn self-discipline.

(Edit: Alternately, he might just propose giving all of the other players pixie dust and non-broom-based flying lessons, to even the playing field that way. Whether that would be accepted, or whether they could even arrange it, is another question - but it does lead to some interesting mental images.)

A couple of tangents from this led me to another potentially interesting idea: since he not only never knew his parents but might very well not care much about them (I think there's considerable variation among the Lost Boys in that respect), what would his Patronus look like?

By further tangent from there - although the Lost Boys' independent mode of flight has several potential advantages over flying on a broom, and so Harry would be likely to disdain the use of one for practical (as opposed to Quidditch) purposes, that independent mode of flight does have one serious relative disadvantage in a certain context: it relies on continuing happy thoughts. What happens when Harry (or, a worse horror to think of, Peter) encounters dementors?

I could see one or more Wizarding factions deciding to exploit Neverland in some way... which could lead to an interesting coalition of "native" forces united to repel the invaders...

Or there's the chance that some of wizards might ally themselves (temporarily, of course) with Hook, by promising to help him take down his nemesis. (They probably wouldn't have much respect for him at first, to have been thwarted for so long by mere children, but that probably wouldn't last past the first couple of encounters.)

I don't think Tinkerbell as a house elf works

Of course not.  She a non-Cornish variety of pixie, obviously.

And either the Cornish pixies are a degraded breed somehow, whose dust (if any) no longer grants the power to fly - or, because of the somewhat specialized requirements for actually using it to fly, it had simply never been discovered to have that peculiarity...