My DD is 9 and in 5th, and is both young for grade and skipped once (from 1st to 3rd). Next year, she'll have an additional subject acceleration into 7th grade math.
In my perfect fantasy world, she'd be classified as a 4th grader in a mixed-age classroom with everyone working at their own level and pace. A grade skip is not as ideal as that fantasy, but that fantasy is not available to us, and all the alternatives that are available to us are even less ideal in one way or another. DD herself has been strongly pro-acceleration, which is one reason it's been a good choice for our family.
Issues so far for us:
We aren't to middle school yet, but DD does feel socially out of step with the boy-girl stuff. I think she'd be somewhat out of step even in an age-appropriate grade, though, due to individual variation.
DD perceives herself as being extremely short, even though she's 55th percentile for her age. She's very relieved not to be the shortest kid in the grade, but she is second-shortest, and probably will be shortest at one point unless she gets her pre-teen growth spurt early.
There are some social / emotional issues that may be maturity, but may just be personality, and it is hard, hard to know what's which. To the extent I had similar issues as a kid, without a skip, and was in my 30s before I figured them out, I'm not going to suggest holding her back in elementary until she figures them out.
But we are working on them in the meantime.
There are some executive function issues that may be maturity, but may be an actual deficit, and it is hard, hard to know what's which. I totally understand now why "young for grade" is such a risk factor for ADD diagnosis. This year, we're getting high expectations for ability to self-organize writing, and low expectations for mechanics. DD really struggles with organization, even compared with agemates, and has exceptional mechanics, and the ability-vs-expectations mismatch is frustrating for her.
It's easy for a kid with the potential to have been skipped to look back on any disappointments in life and point the finger of blame at either having been skipped, or having not been skipped. It's hard to know whether the blame belongs there, or not.
