Down vs. Down's. NDSS and NDSC use the preferred spelling, Down syndrome, rather than Down's syndrome. While Down syndrome is listed in many dictionaries with both popular spellings (with or without an apostrophe s), the preferred usage in the United States is Down syndrome. This is because an “apostrophe s” connotes ownership or possession. Down syndrome is named for the English physician John Langdon Down, who characterized the condition, but did not have it. The AP Stylebook recommends using “Down syndrome” as well.
People with Down syndrome should always be referred to as people first. Instead of “a Down syndrome child,” it should be “a child with Down syndrome.” Also avoid “Down's child” and describing the condition as “Down's,” as in, “He has Down's.”
Down syndrome is a condition or a syndrome, not a disease.
People “have” Down syndrome, they do not “suffer from” it and are not “afflicted by” it.
It is clinically acceptable to say “mental retardation,” but you may want to use the more socially acceptable “cognitive disability” or “cognitive impairment.”
I also came across some myths about people with Down syndrome that may be interesting to those who don't know much about DS. Here is the link if you are interested: