Our school district offers testing from age 3. There is a wide net and a narrow net. In K and pre-K if a child tests at 99.9th percentile they get a permanent gifted label. In K and pre-K if a child tests at >84th percentile they get a temporary "potentially gifted" label and can go into a special program. The latter group must retest in grade 1 (and anyone else can test in grade 1 or later) an must reach the usual 98th percentile to get a permanent gifted label.
It is indeed true that tests at young ages are unreliable, but nevertheless these young kids need a way to move at their own pace right from the beginning.
