Most children make some mistakes as they learn to say new words. A speech sound disorder occurs when mistakes continue past a certain age. Every sound has a different range of ages when the child should make the sound correctly. Speech sound disorders include problems with articulation (making sounds) and phonological processes (sound patterns).
Young children often make speech errors. For instance, many young children sound like they are making a "w" sound for an "r" sound (e.g., "wabbit" for "rabbit") or may leave sounds out of words, such as "nana" for "banana." The child may have an articulation disorder if these errors continue past the expected age.
A phonological process disorder involves patterns of sound errors. For example, substituting all sounds made in the back of the mouth like "k" and "g" for those in the front of the mouth like "t" and "d" (e.g., saying "tup" for "cup" or "das" for "gas").
Not all sound substitutions and omissions are speech errors. Instead, they may be related to a feature of a dialect or accent. For example, speakers of African American Vernacular English (AAVE) may use a "d" sound for a "th" sound (e.g., "dis" for "this"). This is not a speech sound disorder, but rather one of the phonological features of AAVE.
Click link for additional information: