I'm not clear as to whether DA is looking for very bright kids who need an education or very bright ones who already have it.

I think that maybe a bit of both, depending on age. This is what their website says: "Any student who is performing at an advanced middle school grade level or above, meets the Qualification Criteria and is under the age of 18 can apply to The Davidson Academy. It is strongly recommended that applicants have, at minimum, 6th grade in progress at the time of application."
They teach three grade levels above "average" - although they don't group by grades. But, it sounds like they start with a challenging middle-school level curriculum. I may have read on their site (didn't look in depth) that the beginning curriculum is like an average 9th grade one. Anyway, so I think that a student *can* join at a young age, but only if that student has been exposed to enough material that they can join in at the appropriate level.
Our oldest is PG and there's no way she could have scored those 24s or 25s at age 8 (and she was starting 5th grade at that point). She simply hadn't been exposed to the material, had attended a small school with no gifted program, etc...
So, yeah, she would have been one of those kids with the IQ scores >150 and I'm guessing EXPLORE scores in the teens (especially in math, where you can't just absorb by reading).

We still found the EXPLORE helpful, and a low cost way to track our sons' progress, as well as advocate at school, even though we have no intention of moving to Reno if they qualify. smile