I would agree with late middle school, or just straight into high school -- if you do any school at all, of course.

As I've gone through this journey with my kids, starting from knowing nothing and then piling up tons and tons of research and observations, I think that the most crucial time for hsing is up to age 7 or 8, then the next most crucial time is up to age 12 or 13. For the same issues and reasons, but just a matter of degree.
Up to age 12 or 13, kids are just SO different in their development. Not just academically, but socially and emotionally. Once they hit puberty, it really seems to even out a lot. And high school courses are often more general, as well. It's not like how the curriculum difference from grade 3 to grade 4 might be "place value to the hundred thousands" going to "place value to the millions", or whatever. Elementary curricula tend to be nitpicky down to the tiniest minutiae, but still all utterly arbitrary and not necessarily the right fit for any particular kid.
So I believe it's best to follow the individual development as much as possible. Then around the time of puberty, the slower kids have generally caught up to the "norm" -- not always, of course, but generally speaking. Even if there is stuff they haven't yet specifically studied, their brains are now ready to learn it.
So if I were planning to send a child to school, I would do so with that in mind. At the younger ages, they're more likely to struggle with being behind (frustrating) or ahead (boring) -- this it true even if they're in PS from the beginning, by the way, it's not a result of going to PS after homeschooling. Around grade 8, though, they're starting to actually MATURE. So they're ready to handle stresses and difficulties. They're better able to understand the "rules of the game" of grading and exams etc and how to play along for their benefit even if they think it's silly.
And they're got a solid foundation in self-study skills, motivation, etc, so they're ready to work hard to catch up if needed. And in high school especially, there's MUCH more flexibility in course options, so if they're 'ahead' they can just take the next level course.
When my son's father and I divorced (DS was 3 at the time), my ex would only agree to allow me to HS if I had him take a standardized test every year, and we would re-evaluate at grade 4. WHy grade 4? Because that was the year of late-entry french immersion at the time in that province. He didn't want him to miss that entry point. So that's another consideration to look into, if there are any particular school programs you might want your child to participate in that begin at a specific grade.
Grade 4 came and went without a peep from my ex! I told DS that he could decide to go to school for middle school if he wanted, I'd leave that up to him, but we'd HS through elementary. Grade 6 came along, DS said "no way, I don't want anything to do with school" so we're still going strong.
So now I'm telling him, he can decide to go to high school (grade 9 start here) if he wants. He could start in grade 11, too, that's another option -- many universities only look at your marks for grades 11 and 12, for instance. Right now, he still says "no way", so we just take it a year at a time. If he never goes, that's fine by me!!! But he knows he has the option.
But just because he's not in school, he still is getting some of the extracurricular benefits. He's just joined the middle school band program... I just asked. A lot of people had to ask each other if it was alright, and they all said "gee, I can't see why not", so he's in!
