My son was a late talker. I never heard him say Dad until he was close to four. At two and three he had a vocabulary of only about a dozen words and resorted to words and gestures to communicate. But then every so often he would say something in a complete sentence.
Never quite understood what was going on with his slow development.
My other son is 21 months younger. When the younger one began to talk in full sentences, the older one began to talk in full sentences. It was quite an abrupt turnaround for the older one. Obviously the younger one’s language development stimulated the other one.
The older one is still a little behind developmentally. What some kids could do at six, well, he did not accomplish that until he was eight. His skills at driving were not good at 16 and he got his license at 19. He holds a full time job and is getting along though and he never shuts up.
A good non-academic book on this is “Late Talkers” by Thomas Sowell.
SRM