My paternal gran lived with us while I was growing up. She hung on till her late 80's. I'd have been inclined to say it's just the life-force that naturally makes people want to keep going. But in her case, she decided when she turned 50 that she was an old woman, and so she'd do nothing but sit in her armchair all day being waited on. Then when she finally wanted to get up & do things, she found she couldn't manage so well, as her strength, sight & hearing started to fade.
I'll be turning 50 next year, and can't comprehend why anyone would think of themselves as being old at that age.
As a result of looking after my gran, my mum's always said to us that we must put her in an old folk's home when she can't manage for herself. She's about 73, but she doesn't behave like an old person at all. She's always busy gadding about even though she does have some health issues.
I'd look after her if she needed it, as would my sister, and she's got a great network of friends and neighbours who all look out for each other.
If I felt she'd be happier at any stage in a home, we'd look into it, but would be very distrustful of the care she'd get.
As for myself - I dunno. If I could see I was a burden on my kids, I think I'd do away with myself. But I could never think of my own mum as a burden, and I doubt that they'd see me that way either. You don't know till it happens I suppose. When it gets to the stage when my mum needs nursing, I'll just be thinking about how she nurtured me when I was a baby.
Does it sound unkind to wish us each a swift, unexpected death?
It'd be different with my dad though. He showed me little positive attention when I was growing up, so I might put castors on his zimmer-frame.