Following your baby's initial communications through crying, language development continues with first coos, usually experienced between six and eight weeks of age. By roughly eight to nine months, your infant's simple sounds should evolve into vowel sounds and babbling, distinguished by the presence of distinct syllables, such as “dada.” This babbling should increase to include varied consonant and vowel sounds as your infant approaches his first word, typically around twelve months.
As your toddler's vocabulary increases, reaching up to 1,000 words by three years old, she may begin to overextend or underextend the use of those words to new experiences. For example, if your toddler knows that a cow makes the sound “moo,” he might overextend that sound to be used for other animals as well. An example of underextension would be your toddler being able to identify your family pet cat as “cat,” but then not being able to recognize other cats in the same way. Your toddler should also begin forming telegraphic sentences, with simple structures and direct meanings. An example would be “where snack," which is not grammatically correct, yet conveys your toddler's knowledge of basic sentence construction.
Speech continues to develop between years four and eight as your child expands her knowledge of vocabulary, pronunciation, grammar and sentence construction. By six years of age your child should have a vocabulary comprehension of approximately 20,000 words and should have developed the ability to read and retell story plots.
Your child's understanding of language, grammar and syntax becomes increasingly refined, as does his understanding of pragmatics or social language contextual skills. For example, your child should understand that a different type of speech is appropriate when talking to a friend versus talking to a teacher. By age twenty, your adolescent should comprehend a vocabulary of approximately 80,000 words.
Although the majority of language development has occurred by twenty years of age, linguistic skills continue to grow over the course of a lifetime, as new knowledge is accrued through education and experience. This trend typically continues to approximately age seventy, at which time age-related hearing impairment or memory difficulties might begin to impact the continued acquisition of new linguistic material.