Genierose, welcome!
the big thing that a very gifted child gains in acceleration is not learning more academics, because that will probably happen anyway with or without the acceleration, it is learning to work for it!
I agree that in an ideal world, kids with some acceleration would have the opportunity to work hard at learning. In practice, I worry that doesn't occur for really smart kids unless the acceleration is so drastic that there are some serious potential consequences. If a kid needs to be in a high school class at 5 to work at learning, there are many concerns beyond the academic to think about. For most GT kids and especially PG kids, skipping a grade or two or three or five doesn't address the pace of academics, the lack of depth, or that even several grades up, the kid may have already self-taught the bulk of the material.
Genierose, I'd be worried that the academics wouldn't be better for long. I'd also worry that the kid may have challenges that come from development -- issues with handwriting in doing longer papers, stamina to do a whole day and more homework than would be typical for 7, more extracurriculars available and done by others as a social experience that wouldn't be appropriate for a younger kid, and organizational issues if 5th grade involves multiple teachers and multiple different homework requirements. I'd also worry a great deal about bullying and social fit. Keeping the child at grade level for some things like recess, specials, and fun like PE might be worthwhile, but then you have to deal with the kid feeling that he fits nowhere. I'd also worry about the future consequences where the kid ends up out of high school at a very young age and early college or other options have to be considered. Those issues can be addressed later (gap years, undoing skips, etc.) but it's worthwhile imagining them now and thinking about options.
These are hard, hard questions.
