Understanding how children learn to speak takes a framework that explains development in age and ability increments. Speech-language theorist Roger Brown released his stage-defined speech research in his 1973 book "A First Language: The Early Stages." Focusing on morphology -- or word forms -- Brown created a model of language learning that seeks to explain how children acquire and use speech expressively.
Young children who are in Brown's stage one range in age from 12 to 26 months. Between ages 1 and 2 years, children are in early stage one, with late stage I being from 22 to 26 months. During early stage one, toddlers may use single words instead of multi-word phrases. When the young child has built a vocabulary that includes at least 50 to 60 words she is ready to produce stage one sentences, according to speech-language pathologist Caroline Bowen in her article "Brown's Stages of Syntactic and Morphological Development. These include simple, two-word sentences that show operations of reference and semantic relations. For example, a 22-month-old child may say, "More milk" when she means, "I would like more milk now."
By stage two, the child's mean length of utterances -- or MLU -- has grown to between two and 2.5. The MLU refers to the total number of morphemes -- or smallest unit of meaning -- divided by how many utterances the child makes. For example, "quick" is one word with one morpheme, while "quickly" is one word with two morphemes -- "quick" and "-ly." Stage two lasts until roughly 30 months. Children in this stage typically use negatives, such as "no" or "don't" interchangeably, and can complete a sentence that includes a subject and predicate.
Stage three language development in Brown's model lasts to approximately 35 months. During this phase, children begin to use possessives such as "Tom's toys" as well as irregular past tense, according to Bowen. For example, a toddler in this stage may say "Me ran outside." Children also begin to use conjoining words such as "but" and "or" during this stage.
The fourth stage of Brown's model ranges up to 40 months, according to the Center for Speech and Language Pathology. During this stage, children begin using words such as "isn't" and "aren't." With an MLU of 3 to 3.75, children in this stage have a growing vocabulary and are better able to use more complex wording.
Stage V and V+
The final stages of Brown's model are V and V+. The MLU ranges from 3.75 and up for children who are 41 months and over. Children in the last stages can use third person, contractions and indefinite forms such as "no one" and "nobody."