Our DS was not very cooperative at 6.5 when we tested. This wasn't a shock to us given his personality, shyness, etc but we had to test to sort out some things with school. We ended up testing him again a little over 2 years later to get a more accurate picture.
We're waiting until 7 to test DD. She likely would have been cooperative enough at 6 or 6.5 but we've been maxing out our benefits on DS's more pressing issues and it hasn't been as urgent of a need (the school has been way better and recognizing and dealing with her than they ever were with DS even with documentation to back it up...).
One thing to think about is how long schools consider the results valid and the re-testing window. If you are planning on moving to a place that only accepts scores less than a year old and you can't do the same test within a 2 year window for example. Some of our school boards are very picky about which tests they accept and don't accept WPPSI so that is another thing to consider (assuming you are planning to use the scores to get into a school or program).
