We had a friend who is a speech pathologist who asked to test our eldest DS at about 2.7 years old with the Peabody. She mostly works with the other end of the spectrum and was anxious to see him in action. He scored 99.7% and ceilinged out after several hours of testing (she started him really low so it took a long time).
We chose to do formal testing about a year later to advocate for early entrance into K. DS took the SB-V, WRAT-4 (based on 5 year old norms) and the BRACKEN (includes school readiness assessment). His age during this round of testing (3.9 yrs) was an issue that they believe factored into his scores on the SB-V (particularly in the non-verbal area). We will need to retest.
Having the test scores certainly helped us to gain credibility when we were talking to schools, etc. So I would try to wait until you really need the test scores for advocacy/admission purposes. Additionally, your DC needs to have a sufficient attention span/focus, needs to follow directions and be comfortable communicating with a stranger. It is a rigorous process! Our very outgoing DS actually loved the testing process but my more-reserved DH has very negative memories of being tested (multiple times) during his childhood. This has a lot to do with the personality of the tester and the personality of the child.
Anecdotally, DS's % score on the Peabody at 2.7 was exactly the same as his verbal IQ on the SB-V at 3.9.
