Children develop speech sound development in line with their oral motor musculature maturation and according to developmental milestones. These milestones help speech pathologists determine whether a child may need extra help for improved intelligibility of speech.
For example, by the time most children start school they can say all or most sounds clearly in words and sentences and be starting to recognise that spoken sounds can be matched to written letters, hear and make rhyming words, hear and count syllables/beats in words, and make sentences with words that start with the same sound (e.g., big brown bear).