This is a great question and I often pose it to other private therapists that I meet with so I am keeping up to date with the latest. I regularly use norms to point out what is typical development for parents, nursery school teachers and pediatricians. I do notice quite a variance between what different therapists consider “typical development” for expected acquisition of sounds and run into therapists who are working on a sound earlier than I would.
So it was with surprise (and always great respect and admiration) that I read Pam Marshalla’s answer to this question because she has devoted her SLP career to becoming an expert in articulation and phonology. After citing research on vowel and consonant acquisition she makes this statement in her “Speech Pathology Answers and Advice:”
“Using the norms as a basis of deciding when to enroll clients in therapy is a remnant of an earlier age. Stimulability and readiness are more important determining factors today.”
However, she goes on to list the most recent research on norms if you have to use them.
Somehow, I found her advise somewhat liberating in an age of “evidence-based” practice and the pressure to produce research to back therapy decisions.