Well, *I* love reading about all your smart kids! :)
Mine are still pretty young so it's not an issue for us, much. My son is 7, first grade by age, and that's what we call it. He's young for his age, so for outside activities it works well to keep him with his peers. Plus his motor skills lagged a bit. His math, on the other hand ... I've got him in 2nd grade math but really he just skates through it ... I never really know if I should 'skip' him up a level or just let us coast along happily.
I'm figuring we'll start collecting credits in each subject when he hits high school level in each subject, and if we need to stretch things out with dual enrollment and work internships and things, no harm done. We've got several good community colleges and a decent university in our area, so at least there are plenty of options.
I'd also, should we ever have to enroll him in school somewhere, prefer to have him on record as a lower grade, and test out of it, then to think he's in a higher grade but not measure up to whatever the school might require.
But like I said, mine are young, and so we're still kind of newbies at the whole thing. :)