This is not actually accurate, as Special Olympics serves any individuals who have special needs. A quick visit to their website verifies this. There are many participants who might be a typical professional adult who participates in wheelchair sports, a child with a behavioral disability who has been ridiculed on typical sports teams, or someone with a developmental disability.
Also, if you wish to refer to people with mental retardation and/or developmental disabilities, the term you want is "people with mental retardation" or "people with developmental disabilities." If you wish to refer to people with mental illness, the term you want is "people with mental illness." The term "mental disability" is not a proper clinical term, as this is ambiguous. When discussing people with disabilities, "person" (or "child" or "man" or "athlete" or "lawyer") should always come first, followed by the nominal form of their disability ("with diabetes" or "with a disability" or "with paraplegia"), as you don't want to label someone's entire personhood is "retarded" or "disabled."
The APA has a great website on removing bias in language pertaining to disabilities: http://www.apastyle.org/disabilities.html