Many children do not have a serious speech difficulty or impairment (phonological disorder or dyspraxia), but just have speech articulation difficulties with a particular sound, or produce a sound incorrectly for no obvious reason. An example of a common articualtion problem would be a "lisp". When a child has a difficulty with just one or two sounds it is usually not too difficult to improve their speech, although it is advantageous if you try and fix the problem when they are young. If you leave a speech problem till the child gets older, it becomes more difficult to put it right. With speech therapy and practice, these articulation difficulties can soon be fixed. Although there is a general timeframe when we expect most children to be able to produce certain sounds correctly, many children acquire some sounds a little later, but if you are concerned about your child's speech development see your local speech and language therapist / pathologist and discuss the possibility of speech therapy.