We declined to test our DD at age 5, but that had nothing to do with brain development, and everything to do with how our state handles access to GT. She was limited to one test attempt per calendar year, and for a 5yo, access depended on scoring >2.5 SDs, or the 99.5th percentile, on a test of cognitive ability. We declined, because if she failed to hit it on that threshold, we didn't want to limit her access once she turned 6, when the requirements change significantly.
This state sets the threshold this high at 5 as a way to screen out the well-prepared students from the highly-able ones. I hear that 3rd grade is a common cut-off in other places. My advice is to check your state and district policies about access to GT services.
