If they are treated as a child, they act as a child.
If they are treated as an adult, they act like an adult.
If they are treated as if they can't learn anything, they don't learn anything.
If they are treated with the expectation that they will learn, they do learn.
If they are treated as people who are just intellectually disabled, they act as such.
If they are treated as people who think, have opinions and are capable of thinking deeply they do.
I make concerted efforts, when I am instructing people who have intellectual disabilities, to try to stretch them, particularly if I am talking about spiritual things. I am always impressed how they will raise to the level of the discussion. They will often try to take what I am saying and translate it into a direct application to their lives. "So you are saying that I shouldn't listen when somebody tells me to ..." they will say. One gal I know who has down's syndrome, will pause after you ask her a question, and often give profound insights. Too often, however, she is not given the opportunity to do so because the people around her think her pause a lack of understanding, and their limited expectations cause them to be impatient.
I believe I have shared this here before, but I have a friend who has severe intellectual disabilities, lets call him Fred. Fred would try to get my attention by nagging me with a question, the same question over and over again. Finally, one day, he asked me for a dollar. That got me to stop and pay attention to him for a minute. He learned that he could get me to stop by asking me for a dollar. Well many dollars have changed hands over the years, but at some point I stopped and began to have a conversation with him. I expected him to be able to converse with me on a variety of topics. At first our discussions revolved around his original repetitive question and asking for a dollar, but grew to discussion of his desire to marry his teacher, and his brother who lives in Hawaii, and his interest in baseball, and his favorite foods and so forth. When I treated him as a real human being who would communicate with me on a variety of topics, he rose to the occasion. Had I continued in my interactions with him where I basically ignored him, he would have remained something quite less than what he was capable of.
So I have learned to try hard to raise my expectations of people, independent of their level of disability. Too often their low performance is due to what I do as the person who is in control of the social situation. It is the result of mistaken notions of the limitation of the person with disabilities.