And it is children, mostly. Kids start learning language from the instant they're born, and start using it the moment they're physically capable. Have you ever noticed that the words for mother in most languages is really similar? From the English mama to the Arabic ahm to the Punjabi mai, the sound made by the letter m features prominently in most languages' word for mother. This isn't an accident. Ma, along with da and ba are among the first sounds a muchkin can make (oddly, babies make the g and k sounds during the "babbling" phase of vocal development, before they have any real control of their vocal tract, but they lose the ability to make those sounds when they're a few months old, and don't get it back until many months after that). Of course, you could say that while babies might make the sounds, it is the adults who assign meaning to them, and that's true to some degree. But even without adults around to communicate word meaning, kids do a pretty amazing job of word creation on their own.
One case that Pinker uses to support his point is that of Nicaraguan sign language. Before the Sandinista government took over there in 1979, the country had no schools for the deaf - so no universal sign language existed. Deaf folks communicated with their hearing friends and family with their own attempts at sign language, but it wasn't until deaf kids got together that a fully functioning sign-language was born.
|And anatomically correct|