The lisp is one problem that Speech-Language Pathologists have trouble agreeing on and the following is my take on lisps:
I would be very surprised if you haven't noticed that your toddler used to, or currently does stick his tongue out of his mouth when saying the S sound. My daughter certainly does! Infants have a different swallow pattern than the rest of us do. To accomodate the different feeding style (suck and swallow) an infants tongue pushes forward when he swallows, instead of moving back. Thoughts are that a lisp in the toddler years is a remnant of the forward tongue motion of a baby's swallow. Most toddler's grow out of a lisp without any trouble; however, there is the odd child who has a hard time getting rid of this pattern. I go by the rule of thumb that if a child hasn't started to say a nice S sound by age 4 or 4 1/2, they're headed towards a very difficult habit to break. I tend to offer therapy around age 4 with the hopes that a child won't enter school still talking with that pesky lisp.
Where does the controversy come in? On one hand some therapists feel that a lisp at any age is not typical and will offer therapy at a younger age than I typically do, on the other hand, some therapists work with too many children to bother treating a sound that doesn't look right, but doesn't affect how clearly a child speaks.
As children get older and more aware, lisping can affect self-esteem because of teasing and the social stigma associated with having a lisp. For this reason, I hope that all children who want help have the opportunity to get help.