Hmmm...Very thought-provoking post, J.
I, too, like the more global approach that CFK and Isa suggest, but am having a wee bit of trouble with the straight "What can they show us?" of this age/grade level scale idea.
The info GT kids choose to pursue does seem relevant to the discussion and gets left out of the "What grade level are they operating at?" approach. The intensity of the interest also matters, and I think that intense but untraditional interests get left out altogether when looking at age/grade level alone.
For example, liking cars is one thing, and may even be age-appropriate for a 2yo. But at at age 2, knowing EVERYTHING about cars--how they work, the makes and models of every car on the road, etc.--is not age-appropriate (!) and it does indicate a depth of interest and ability that wouldn't show itself in testing or in this above-age-level strategy. I'm solidly into middle-age, and I haven't learned these things about cars yet, but my DS did well before his 3rd birthday! I used to joke that if we needed to ID a getaway car, I'd have to turn to my toddler and say, "What kind of car was it, honey?" How'd that be for a witness!?
I don't mean to be just a wet blanket here... I'm afraid I have no helpful suggestions about how to catch these kids whose interests are so far off the beaten path. *sigh* I suspect that if there were an easy, straightforward way to ID these kids, someone smarter than me would have found a way to do it long ago!
