I think that the other thing to consider is that achievement tests don't give a full picture of the child's functioning. Dd#1, for example, took the WJ-III , the SRI Lexile, and the DORA (some kind of reading test) in 2nd grade when she was 7. On the written expression & word decoding parts of the WJ-III, she scored in grade 18+. On the DORA, she was at grade 12+ in numerous areas and on the SRI Lexile, at grade 7 or 8, I believe.
None of these addressed readiness for reading content of middle or high school level books &, while dd is a really good writer, I really don't think that she could write as well as a typical graduate student at age 7 in terms of syntax, grammar, ability to write research papers, etc. Writing achievement tests, in particular, don't usually give you a good feel for whether the child can write the type of material (research papers, persuasive arguements) that would be expected of older teens. I believe that the written part of the WJ that dd tested so well on was more related to how well she could construct a basic sentence.
