Arnie

developmental delay

He's supposed to have a congenital "mental redardation." I forget the percentage, but I think about 50% of cases of developmental delays that are present since birth are caused by an unknown reason. (Ya know, as apposed to something like Down Syndrome or Fragile X that can be easily diagnosed.) It would seem doctors erroneously assumed Arnie wouldn't survive long for some reason.

I don't know if the book explains this, but the movie offers no reason as to why doctors would assume Arnie would pass away at a young age. A developmental delay on its own doesn't cause shortened life expectancy, but some causes do also result in other health problems. (Like heart defects or bone marrow abbormalies being common in Down Syndrome.) It seems Arnie had nothing else wrong. Maybe it was just small town doctors not knowing any better when he was botn in the early 1970s, and then his family continued to hold onto that original info given, worrying he wouldn't live long.