As a speech-language pathologist, I am often sought after by friends and family to provide solid information on development. My go-to responses often include: one year-one word, two-word combinations by 18 months, and walking around a year. Early developing sounds include all vowels and consonants /b, p, m, t, d, k, g/ (within the first 2-3 years).
However, this is not a perfect science. In fact, there are several schools of thought in the research about the when of sound development. The Virginia Department of Education uses the Iowa Nebraska Articulation Norms in schools as a guide but then the Goldman-Fristoe Test of Articulation Norms present some discrepancies.
So what do we do? This seems far from NORMal. My rule of thumb is often to use these as a guide. When testing children, we need to have some kind of reference and we can think about early (those named above) vs. late (“th” and “r”) developing sounds.
Another important “milestone” that is a good measure of sound development comes within a child’s intelligibility. I usually think of the following numbers when it comes to intelligibility to an unfamiliar listener (parents and teachers are deemed “familiar listeners”) when context is unknown: by 2 years old a child should be about 50% intelligible; by 3 years old a child should be about 75% intelligible; and by 4 years old a child should be about 100% intelligible. Keep in mind that I don’t think anyone is always 100% intelligible – this will specifically happen on the phone, in a crowded and noisy room, or when someone mumbles. Remember, these numbers are just a guide to get a general idea, as all children develop differently.
In any case, I always recommend visiting a licensed speech-language pathologist to assess articulation if you have the slightest concern. “Mother knows best,” as they say so go with your instincts. It’s always better to be safe than sorry!