Not all sounds are created equal
The other issue for Individuolect period is the child's phoneme pronunciation. By four years of age, the child is a master of English. At least as far as the morphology, syntax, semantics and pragmatics are concerned.
Ironically, the mechanism upon which all these ride for expressive language may not yet be complete. Not all sounds are equal and hence not all sounds are learned at the same age.
Some are easy to see and require large muscle action, like /p/ and /m/. These you might expect the child to be able to make by age three. On the other hand, /r/, /l/ and /s/, for example are difficult to see and require complex motor adjustments of the tongue to produce.
It is not uncommon for children to acquire these sounds after five years of age.
We have no problem! So, when little 3 year old Edward says, "I thee the puthy cat thitting on the fenth." is there cause for alarm?
It all depends on the sound and the age of the child. In this case, for example, is the mispronounced sound, /s/, one that is typically is mastered by a three year old child?