Yes, something in the 6-8yo range is what I've heard. FWIW...No official pronouncement, though. This is just what "they" say.
Testing was pretty useful for us. Not only did it get DS7 into DYS, which has been good for many reasons, but it also helped me to know about a bottleneck in his learning that has changed the way I teach him.
Different kids are different, and much depends on what the test results for your specific kids are, of course. But I always recommend that if you have a reason to test beyond just mere curiosity, and if you have the money to test, and if you can find a good tester...well, then you should probably test.