We tested DS at 3.5 (just this year) - we started to question public schools due to DS's emotional sensitivity since he seems to feel things more intensely than his classmates and yet he is one of those "sweet calm boy" as all his teachers say about him... And we were seeing flashes here and there where he seemed to break way out of what we expected a 3 year old would say or worry about. DH and my families had history of giftedness (verified and unverified) along with LDs so we wanted to get an early idea of what issues we may be dealing with and whether we needed to look at other school options... Especially since we noticed that he seems bored with preschool and seems to be out of sync with his current peers. His motor skills are completely age level and he looks like such a typical little 3 year old including his articulation... and unless you catch him in an area that he is passionate about, we noticed that he already seems to be playing down his knowledge and abilities.
Although we are still struggling with what to do, we are glad we did it since we were still unsure he would fall into the gifted range and were surprised by the results especially with his verbal being so high (and it looks like we really have no idea what a 3 year old would be expected to know - makes me wonder if we accidentally gave presents esp books that were too advanced to his classmates at times).