It was pretty clear to us by the time DD was oneish. I'd say we both noticed at the same time. She was a very precocious toddler, though, and looks a little less "out there" now. Her ped also said something about it at her 1-year visit.
With DS, we had the expectation that he would be similar. His early development was slower and his speech was not as startling as DD's, though still very precocious. His fine motor skills well outpace hers at this age. I'm still not sure how he will test. He is not as obvious as DD.
