It's been a while since I read the book or read the websites. I don't remember the general range of when these kids start talking. I do remember that some were diagnosed as PDD-NOS when the parents felt that it was an incorrect label assignment. These are very bright kids (gifted never mentioned) who lack expressive speech but possess no other developmental markers. Here is my vague recollection of it, in addition to my son’s speech development timeline.
In our case, our son had no words until about 20something months (I think it was 22 months), except for "Dada" and "dissie" (i.e. nursie). He had never even said, "Mama"! He did babble, but generally it sounded like "da da da da". He grunted and pointed a lot. He could be very expressive when he babbled, like his voice would go up and down and he made eye contact; it was just, "DA dada da daDA".
What was clear to us (but not to our ex-ped) is that he had a very large receptive vocabulary. When he was 15 months (which is when our ex-ped started pressuring for an eval, if you can believe that), our son could sit with a picture book that he had never seen before and point to any requested object (e.g. where are the curtains?) It was an important clue that he clearly could understand what people were saying.
Around 18 or 19 months, when the ex-ped's pressure greatly increased, I bought foam bath letters and I started quizzing ds1 on sounds to rule out things like: 1. hearing issues, 2. ability to cognitively understand what I was saying to him and 3. ability to make isolated sounds in the mouth. That's how I accidentally taught him how to read uppercase letters. I quizzed him on each letter a handful of times (and found that he could repeat all sounds except hard g and k) and he learned them. He started pointing to letters everywhere we went. He started poring over adult books to search for his favorite letters ("buh" and "huh"). He could identify them anywhere on the keyboard, he could identify them in small print on the back of a Hallmark card and he could identify them sideways and upside down. I know, because I was freaked out about it and so I asked him a lot.
In addition to child development books, I read the Einstein book somewhere around that time, after someone recommended it to me. IIRC, the book consisted of a combination of anecdotal case studies, profiles of famous people and general surveys of families. The general surveys (by Prof Camarata) revealed general tendencies of these kids. These are kids, IIRC, who have a large receptive vocabulary and appear completely normal but simply do not speak. I don't remember how many of them said how many words or how long they generally go on for without speaking. But they just lack expressive speech. They also tend to exhibit preferences for things like puzzles and mechanical things. In retrospect, it seems to describe a very classic visual-spatial learning style. There is possible overlap with autistic tendencies, like great affinity for patterns; there is overlap between gifted and autistic as well in many cases.
The other thing that jumped out is that these kids tend to have multiple people in their immediate families who work in highly technical professions, in addition to having musical tendencies. In our case, my husband is a computer programmer and general mathie, my FIL was a finance manager and is a whiz with numbers, my father is one of those people who can fix anything just by figuring it out and my SIL plays several instruments. The book cites many famous mathematicians, scientists and the like (many of whom were also musical) who also spoke late. Oh, and most of the late-talkers were male, IIRC. They were usually the only child in a family who spoke late.
The book seemed to talk about “left brain/right brain” stuff. The idea was that these kids have the typical ability to speak, but that devote their mental energy in the toddler years to other things (puzzles, mechanical things, etc); they effectively neglect expressive speech development in favor of advancing technical gifts. My son at 2, for example, could recreate Lego creations based on something my husband showed him the night before. He could solve 100 piece puzzles by 3. He seemed to spend much of his time focused on things that other toddlers weren’t into yet. And the other toddlers, of course, could speak because that’s what they were focused on.
I’m looking at the baby book/family website now and my son’s speech development looked like this:
**Prior to 22 months – Said Dada, dissie, and babbling, but didn’t say Mama. He was very serious and precise; he attracted people’s attention because of his silent and intense demeanor. Learned all uppercase sounds and was obsessed with finding the letters in the world around him that matched the sounds.
**Right before 22 months – I gave him something and he replied “tay-too”. The next day, he asked for “wabwa (water)”. I was beyond thrilled. Ped, of course, didn’t think it was enough, because other kids ds’ age said much more than that.
**22 months – We went to England to visit family. I don’t know what happened, but the flood-gates opened and he started repeating words we said. He didn’t use the words to express himself yet, but he just repeated certain words if we asked him (including radiator, which sounded like “yay-yee-yator”.) He said “Mama” for the first time.
**23 months – Started using isolated words to express wants, mostly food words and “outside?”
**24 months – Started saying small phrases, like “Ooo, vat is dat?” Learned to say “no”. Prior to this, he used to strike his chest to indicate "no".
**25 months – Starts singing very small parts of nursery rhymes. Reads his first word “stop” after seeing a white sign that says “stop here on red.”
**26 months – Says longer phrases like, “Mama sit down over there.” Starts accurately guessing which words say what in “Goodnight Moon”. Says trapezoid which sounds like “AH-uh-zoy” and points to one. Finishes a counting sequence like “101, 102, what comes next?”
**27 months – I estimated his expressive vocabulary to consist of 400-500 words, including “pentagon” and “nostrils”.
He was late talker, but we did not consider him to have a true speech delay. He never attended therapy and once he began talking, it came on very abruptly. It did not affect his reading (someone asked about that) because his speech quickly caught up in the late toddler years. In addition, the fact that he had a very large receptive vocabulary and general exposure to big words meant that reading did not really help him acquire words. He had words in his head, but he did not say them.
The Einstein book contained many anecdotal sections on various children, but I just don’t remember what the variation was. I do remember that at least one of the famous people profiled didn't speak until age 4. I think that, for many, ages 2 and 3 were common ages to begin speaking.