Hi BaseballDad - our DS5 tested on the SB-V in the spring at age 4y1m, because we wondered about early k entrance and we'd have to have all the paperwork the spring before school started. He tested very high, and the psychologist recommended letting him play another year, since accommodations will be needed throughout his education. Because we were armed with this info, we contacted the gifted coordinator at our local public school recently, and she arranged for WJ-III testing and a school team met with us about what to do next year. I think DS was 5yr2mo when he took the WJ-III, and he did very well on that too. The psychologist gave him some tests that are only normed for older kids, so we have an idea of what level he is, but we do not have complete, exact information from the WJ because you have to be 6 to get all the tests normed.
Because they pick teachers in the spring, they want to be able to hand-pick a good teacher ahead of time, so I'm glad we had all this info in advance. I think the detailed SB-V report made a big impression on the school, and they have been extremely helpful. And we got a free WJ-III test out of it!
So, those were our reasons for testing before age 6, and it has worked out very well for us.