"That Down syndrome boy."
"That girl has Down syndrome."
Which one is correct? The second one, "That girl has Down syndrome." Always put the person before the syndrome. People "have" Down syndrome, they do not "suffer from" it. Those with disabilities are not defined by their disabilities, which is why their disability should not come first.
This is Octavia. Octavia is smart, funny, loving, sweet, and kind. Octavia has Down syndrome. Octavia is not Down syndrome.
Also, the term "intellectual disability" has replaced "mental retardation" as the appropriate term. Many advocates for those with special needs condemn the use of the word "retard" or "retarded" in any derogatory context. Using either of this words, in any context, is hurtful and suggests that people with disabilities are not competent.