My MIL isn't even elderly! She's 64 and in good health. She has a spare bedroom that is just full of junk. There is hardly a path to walk into the room.
I am far from a neat freak, in fact I can live in hearty denial about the state of some clutter (like too many books or messy closets/drawers, etc.), but once I snap out of the denial I can't relax until I do something about the clutter. I guess MIL/SIL/BIL have an even heartier sense of denial then I do, because it would drive me crazy looking at that junk every day.
I've had this problem with a certain relative.
It starts with the foot-tall stacks of "important papers", continues to the pens and pencils that I know have been in that pencil holder for the last 20+ years, and then right down to an infatuation with the most trivial of personal possessions (e.g., a $20 entrance mat from K-Mart).
During the garage sale, several items almost but didn't sell because of a $2 or $5 dispute. Stuff that will ONLY go in a yard sale. So instead of getting things cleared-up, there is the need for yet another yard sale.
It took more than a year to get the accumulated junk cleared out.
It was a test, since my own spring cleanings are utterly ruthless.