Comment by ta1243
My nan made it to 92 without any mental issues, but then deteriorated significantly over the course of 18 months, forgetting she'd ever been married, had kids, etc, just reverted to believing she was a teenager who wanted to go home to her parents (in a house which was destroyed in ww2)
She couldn't look after herself was was forced into care by the courts. Since going into a home she's physically never been fitter, but mentally she's not the person she was 10 years ago -- it's not that she's changed personality, it's as if her memory of the last 80 years was wiped.
That’s terrible, but you never know when or if the decline is going to happen, so if you pick an arbitrary cutoff you’ll have killed people who had plenty of good years left.
Many people die suddenly with no decline at all.