Do older children or adults respond to this diet?

Yes. Obviously, the younger the child the more likely he is to "achieve normal functioning." However, many older children and adults have made dramatic and remarkable improvements, not just in their level of functioning, but in their sleeping patterns, anxiety levels, and comfort levels.
Other factors seem to include current level of functioning, condition of the gut, immune function, and of course, the subtype of the disorder. We have heard from high-functioning adults with autism who describe going on the diet as "a fog lifting." Clearly, most older children have not been "cured," but many have achieved functioning far beyond their caregivers' expectations.