Since all children develop at slightly different rates, it can be difficult for parents to determine when a child might need extra help. As a general rule, however, if parents are struggling to understand their child, or if they often find themselves acting as their child's interpreter, then a speech evaluation is warranted.
Based on my 16 years working as a certified speech-language pathologist, and considering published developmental norms, I would like to share my personal recommendations for when I like to treat specific speech sound errors.
Age: Work on sounds:
As an experienced private practice speech therapist, my preference is to treat a sound before the error is so ingrained that it is difficult to change. Most children can learn to say the above sounds by the ages indicated, unless there are additional physical or cognitive constraints.
Having worked in the public school system in Utah for over nine years, I am aware that students may not be eligible for school-based speech therapy until much later - for example, age 7-8 for "th," "r," or "s" errors. However, in my private practice I have the luxury of working with children considerably earlier. If a child is physically and cognitively able to acquire new sounds, I prefer to intervene before error patterns have been solidified for several years and negative social consequences have been encountered.
A Word about R
It is very common for 4 and 5-year olds to develop the "r" sound on their own. In my experience, however, beyond age six, spontaneous remediation is less common and treatment is more likely to be indicated as the child ages. That is, 4 and 5-year olds often learn "r" without treatment; 6-year olds sometimes learn "r" without treatment; and 7-year olds (and older) rarely learn "r" without treatment.
During evaluation or treatment sessions, an experienced speech therapist can often tell if a child is "close" to learning a particular speech sound by checking for what we call "stimulability" of the target sound. A child for whom a speech sound cannot be stimulated (that is, produced correctly with prompting) is less likely to develop the sound on his own in the near future than a child for whom the sound can be "stimulated" easily.
Hopefully this helps as you consider your child's speech development and whether or not to pursue evaluation.
-Jayna Collingridge, M.S., CCC-SLP