Most children develop speech production or articulation skills and are able to produce all the sounds accurately by about age 7 or 8 years. Children follow a general order in the acquisition of sounds in English, starting with the sounds which are most visible like /b/, /p/, and /m/ to the sounds which are less visible and more complicated to produce like /r/, /th/ and /sh/.
The chart below shows the age by which 85% of children have mastered producing a sound in a specific word position. Many children develop these articulation skills quite a bit earlier since there is a wide range of normal development.