Individual sounds and the pieces of words that make up language (phenomes) typically develop by a particular and 'normal' range of age. If a child hasn't reached the developmental milestone for a particular sound it is possible they could have an articulation disorder. Common evidence of an articulation disorder in children includes the omission (saying 'at' instead of 'cat'), substitution (saying 'fat' instead of 'cat'), addition (saying 'raceamacar' instead of 'racecar') and the distortion (saying 'thracecar' instead of 'racecar') of sounds. These sound mixups are common in all children up to a certain point (and contribute to how cute kids are when learning to speak!) but if they are still present after the appropriate age has passed then it is a good idea to have a doctor or psychologist examine the child to ensure there are no auditory or cognitive issues at work.