A nine-month-old child is typically developing if he can speak even one word. With the benefit of proper scaffolding, he'll know fifty to one hundred words within just a few months. By two, he will speak around 320 words; a couple months later over 570. Then the floodgates open. By three, he'll likely be speaking in full sentences. By the time he's off to kindergarten, he may easily have a vocabulary of over 10,000 words.
For years, the advice has been that the way to kick-start a child's language learning was to simply expose kids to massive amounts of language. However, as we explain in our book "NurtureShock: New Thinking About Children," the newest science has concluded that the