Developmental researchers have learned a great deal about the order in which children learn different aspects of language. Their first words are almost always nouns. Verbs come later. Early “sentences” consist of only 1 word. Then comes the 2 word stage. Etc. These stages tend to happen at particular ages.
We know much less about how children learn language. For instance, just because children typically produce one word sentences first doesn’t mean that it’s a necessary step (in fact, some children appear to only start speaking when they are capable of producing multi-word sentences). Maybe the classic developmental trajectory doesn’t say anything about how language must be learned, but instead says a lot more about what babies at different ages are able developmentally able to do. Thus, maybe 1 year olds speak like 1 year olds as opposed to 3 year olds not because they only have 1 year of experience but because their brains are only 1 year old.
One early interesting “experiment” involved the discovery of a severely abused 6-year-old by the name of “Isabelle.” She was locked in her attic by her mother and apparently never spoken to. With a year of being discovered and rescued, she was able to speak at the level of her 7-year-old peers and even started an ordinary school. This was pretty good evidence that the slow pace at which babies learn language may have more to do with their brains than the nature of learning language.
Unfortunately for researchers, but luckily for children, cases like Isabelle are very rare, limiting how much research can be done. Members of our lab were able to discover another way of doing this research: cross-linguistic adoption. Many babies and young children immigrate to the USA each year as adoptees, typically from the former Soviet Union or from China. When they come to the US, they typically are no longer exposed to their home language (even when their adoptive parents try to learn the baby’s original language, they often learn the wrong language — Mandarin instead of Fukinese or Russian instead of Ukranian, for instance). If caught at the right age, before they have learned much of whatever their home language was, they are excellent case studies in how language develops if you start at, say, 3 years old instead of Day 1.
The results of this study (full disclosure: I was not involved in this study) are that these children seem to go through all the typical stages of language development, just much, much faster. They very quickly catch up to their American-born peers. Which is good news for them, and tells us a great deal about how language develops.