By approximately preschool age (4 years old), what is the highest stage of language development that most children have mastered?

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By approximately preschool age (4 years old), most children have mastered most of the syntactic rules of their language. They can form complex sentences, use proper grammar, and have a vocabulary of approximately 1500-2000 words. While they may still make some errors and need to learn more complex vocabulary and grammar in the coming years, the highest stage of language development they typically reach at this age is mastering most of the syntactic rules. Overextension (A) describes the tendency to apply a word too broadly (e.g. calling all animals "doggy"), which is common in toddlers but typically decreases by preschool age. Two-word sentences (B) are typical of early language development but are typically surpassed by preschool age. Some application of morphological markers (D) is also typical of preschool age, but is not necessarily the highest stage of language development at this age.

By Approximately Preschool Age (4 Years Old), What Is the Highest