Articulation is the production of individual speech sounds using your tongue, lips, teeth, jaw, and palate.

  Pre-K children should be able to be understood 90% of the time to unfamiliar listeners.
  Kindergarten children should be able to be understood 90-95% of the time to unfamiliar listeners. 

When a student is unable to produce an age-expected sound/s correctly, it is called an articulation disorder. Below is are charts illustrating at what age a sound should be mastered. Sounds develop at different times for boys and girls.

Here is a great tutorial on how to produce the S sound!