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Recent research has shown that language is integral in such brain functions as memory, abstract thinking, and, fascinatingly, self-awareness. Language has been shown to literally be the “device driver”, so to speak, that drives much of the brain’s core “hardware”.

Thus, deaf people who aren’t identified as such very young or that live in places where they aren’t able to be taught sign language, will be significantly handicapped mentally until they learn a structured language, even though there is nothing actually wrong with their brains. The problem is even more severe than it may appear at first because of how important language is to the early stages of development of the brain.

Those completely deaf people who are taught no sign language until later life will often have learning problems that stick with them throughout their lives, even after they have eventually learned a particular sign language.

Source: "How deaf people think" from 'Today I found out'--more fascinating tidbits in the article, check it out

Two related, equally interesting articles from 'Today I found out':

A woman who lost the ability to smell, taste, see, and hear as a child was the first deaf blind person to be fully educated

Helen Keller was not born blind or deaf