A hearing threshold of 30dB or more for most frequencies in both ears would cause a significant speech and language problem
I have known some young children to test as low as (minus) -30dB!
As a very gross rule of thumb (to serve our purposes here in describing speech and language development) I am going to say that any hearing threshold 20dB or below is within the normal range.
Also, (as a gross rule of thumb) any threshold 30dB or greater will pose a significant problem for language/speech development and use.
That is, providing the loss is for most frequencies in both ears. You can develop speech and language in a normal fashion with just one good ear.
So what are the chances that a person would have a 30db loss in most frequencies of both ears? Unfortunately very good!
The highest incidence of hearing loss in the United States is due to middle ear infections, which are very common in young children.