Well, my DS is just 22 months old, so we don't "officially" know (in terms of testing), but I'm certain he's gifted. In retrospect, there were signs from the very beginning - even the delivery room nurses commented on how alert he was - but for his first year, I mostly just thought he was very physically coordinated since he was very advanced on gross/fine motor skills (lifted head at 1 day old, sat unassisted at 3.5 months, walking at 8 months, etc). I figured "well, he's an early walker so he'll be a late talker" (I'd heard that somewhere).
He started talking at about 11 months, not exceptionally early, and initially I didn't think much of it. My first real "is there something going on here?" moment was at 14 months when he randomly flipped open a colour book and correctly pointed out yellow. I thought it was a fluke at first, but within a month or so he had the basic colours (yellow, red, green, blue) down pat and was starting to learn and point out letters and numbers.
By 18 months, he had about a 300 word vocabulary (I tried listing, but gave up after 250), knew 10-12 colours, recognized all the letters and numbers up to 9, could rote count to 12 and was starting to do one-to-one counting up to three or four. By that point, I KNEW something was out of the ordinary and started doing more research, which led me here.
