As children, we learn consonants developmentally. We begin with our ‘early developing sounds’, like ‘m’ and ‘b’, and practice them in babble. As we grow, we add more and more consonants to that inventory and our babble slowly turns into words. 

Some sounds are very difficult to produce, so our muscles simplify them for us. You can see this in children who say ‘wed’ instead of ‘red’ or ‘mouf’ instead of ‘mouth’. By the age of 5, we don’t expect a child to have mastered all of the sounds, but we should expect that they are totally intelligible to others. By the age of 6, a child should have mastered all of the sounds in our language.

Sometimes, those simplified patterns don’t budge and children continue to use error sounds long after they should have mastered the sound. Conversely, sometimes a child can produce the sound by itself or at the start of a single word, but when they try to put the sound into a sentence or story, they revert back to their old error. This is not uncommon, and it requires therapy to break the error and form a new pattern.