Articulation, or speech, refers to the sounds produced that make up a language.
Typically, young speakers of Standard American English produce most vowels and consonants such as /b, d, m, and p/ by 36 months. Oftentimes these sounds are heard in early babble such as when a baby produces "mama" or "dada," and can be produced as young as 10 months.
However, the three sounds that are most often mispronounced by ages five and six are the /s/ (e.g., the tongue may protrude between the teeth as in a lisp), /r/ (e.g., the child may substitute /w/ for /r/ as in "wed" for "red"), and lastly, /th/ (e.g., "thumb" may sound like "fum").
While some children may not accurately produce these sounds until age 8, most children accurately produce all sounds by age six.