Our oldest son, now nine years old, is in the gifted program in our local public school. He was the youngest child ever tested (kindergarten) and when the testing was done, it was found that he was reading at a sixth grade level! His reading level now is college level.
His quirks, now that I can look back on them, were many. At 18 months you could ask him to go get the letter "A" from the refrigerator (magnetic letters) and he would. He knew all his letters at that age and by the time he was two he knew the colors, shapes, etc. I thought this was normal since he was our first one! He was obsessive about his quest for knowledge and has only in about the last year or two quit asking, "Why, why, why?" He went through what I called phases of doing stuff like coughing repeatedly, doing flicking motions, chewing on his fingernails, chewing on his toenails...all for about six months, no matter what I did to curtail them.
I suppose if he were in public school he might have been labeled ADHD but I wouldn't have medicated him if he were. His activity level and inquisitiveness, though they surely are a trial to me, are WHO HE IS. He is attending a small Christian school right now who work with him keeping him stimulated and busy. If that eventually does not work I'll homeschool him.
I don't know if I've helped or not but these are the things I've noticed with our son. We have a daughter who is totally normal and another younger son who doesn't show the same traits that the older one does. And you know what, that's just fine.