I don't think it is inconceivable that the adult autism sufferers remain undetected. Presumably, their lives are already arranged. Either they survived somehow, or they didn't. If they survived, there probably is some kind of routine that keeps them alive (maybe an institution, or they live as beggars on the street, or whatever). In that case, why should somebody check up on them again? Sure, it would be nice if somebody would, but why should it happen? When was the last time you learned about some new medical breakthrough and thought "hey, let's go out on the street and check all homeless people to see if they have this condition"? (Not saying older autistic people are homeless, I have no idea how they survive once their parents are dead). Also, there is no cure for autism (or is there?), so simply correcting the diagnosis might not help the older autistic people much (at least not "help" in terms of making it easier for the people they live with - might be easier for themselves, though). Maybe now they are simply sedated with powerful drugs most of the time and don't do much.
Anyway, the main point remains: be careful with "common sense" conclusions.
Anyway, the main point remains: be careful with "common sense" conclusions.