I was told by my local school district that the appropriate age for giftedness testing were between the ages of seven and nine years old - which is the age at which the results would be "reliable". If there were no other concerns, they wait until kids are 8 years old in the 3rd grade to test. For a lot of kids with none to low exposure to reading and language, it is true that testing might be better after a few years of schooling.

In our case, we tested earlier privately because we did not want to wait for the system to work for our child.