The book "The Einstein Syndrome" is very good--it discusses how a small percentage of late talking children are very intelligent (tend to be advanced with math, puzzles, etc),and eventually they catch up. My DS was one of those. We could tell he was very bright. He could name all the letters, numbers, etc. by age 2, he could do complicated jigsaw puzzles, etc. But he didn't speak in phrases til 2.5 and even at 3 he was very quiet, didn't ask many questions, didn't always respond appropriately to questions like "why" or "how". Having any sort of reasonable conversation with him was pretty much impossible. But he did not appear to have autism (despite having some autistic traits like toe-walking and hand-flapping and speaking in an odd voice). I took him into a developmental pediatrician when he was 3, who referred us to a psych, and she did an IQ test. He was average in verbal IQ and somewhat above average in performance IQ. They declared him "developmentally delayed". He also had some delays in gross and fine motor skills, for instance he couldn't hop up and down, jump forward very far, walk on a line, hold a pencil with a decent grasp, etc.
Fast forward a few years and now he is 6. His verbal and non-verbal IQ both went up about 25 to 30 points. But there is a 27 point gap between the two. He is 99.7 percentile for non-verbal and 80 something percentile for verbal.His overall IQ is in the 99th percentile. So my speech delayed kid who I was worried was cognitively impaired at age 2 is now doing quite well. He does have developmental coordination disorder, however, which still affects his speech and motor skills. His conversational abilities are fine, but he struggles with fluency and prosody. His speech strikes people as a bit odd. He has an "output" problem similar to apraxia of speech. But is making good improvements despite that.
