Children develop their speech production skills (i.e., articulation) at varying rates. Some children may be very easy to understand once they begin putting words together and other children, even in the same family, may have difficulty pronouncing certain sounds. It is important to remember that there is a range of typical development for all skills that children acquire, articulation included. According to various studies, children between the ages of 3 and 4 should be beginning to be more easily understood by people who are not in their immediate family; studies suggest that three-year-olds should be understood by unfamiliar adults at least 50-75% of the time and that by the age of four, preschoolers should be understood most of the time by unfamiliar adults. Four to five-year-old children are able to say many sounds correctly but may still have difficulty pronouncing more difficult sounds including l, s, r, v, z, ch, sh, and th. These consonant sounds are later developing and continue to develop for children through the early elementary grades.

Please open this link to see a visual representation of the average age of acquisition of consonant sounds: Age of Customary Consonant Production

from American Speech and Hearing Association: Three to Four Years and Four to Five Years

from: American Speech and Hearing Association, Your Child's Communication: Kindergarten and Your Child's Communication: First Grade