This is a horrible, yet interesting question, as it's something that is bothering me, yet my brother/sister do not want to seem to want to think about it.
My dad is 76, incredibly frail to look at yet tough as old boots (does driving for meals on wheels and refers to the peeps he delivers to as 'the geriatrics' even though some of them are younger than him... ). However, he is becoming more and more forgetful, to a frightening degree and, being an evil-tempered git (one thing that has not dimmed with age!) will argue violently rather than admit he has forgotten something.
Mum is 70, had a mini-stroke 4 years ago and, although she has made a full recovery, is trying to be realistic about the future and talk to Dad about putting the house into my brother's name now (he lives with them), so that if either of them have to go into care in years to come, his home will be safe.
She nursed her mum through senile dementia and has repeatedly said she would not wish that on anyone and we are to put her in a home at the first sign of doolallyness (as she puts it).
Dad goes mental at the mere mention of this, 'they're not putting me away' etc etc, but the fact remains he is such a cantankerous, downright nasty, violently-tempered sod (and that's being polite about him) who terrorised our childhoods, none of us want him anywhere near us and my sister and I are certainly not prepared to have him come and live with us. It would be the end of my marriage, for sure.
If it were not for my mum I think that my brother and he would have killed each other by now, so if she goes first God alone knows what we'll do....
In the meantime, my brother seems to have buried his head in the sand vis a vis the possiblity of him being left to look after Dad/Mum in their dotage, and my sister is telling me that I'm exaggerating the problem - given that, the week after Mum's 70th, the entire family up for a big birthday lunch in posh restaurant, major fuss, extravagant presents and surprise cake, he turned to her and said - oh, sorry, I forgot your birthday, didn't I? When Mum replied no, you got me xyz, we all went out for lunch, don't you remember, he said - oh, did we treat db to the pub, then? - I don't think that I am exaggerating too much...
In terms of my own dd, I don't expect her to look after me. I'd hate for her to see me the way that I saw my grandmother in her final years, it was very scary, esp. as she thought that I was her eldest child and this b&b we'd been staying in was very nice (my parent's house!), but we really must get back to our own house (her terrace in Liverpool which had been flattened in the Blitz some 20 years before). She kept trying to get me dressed in the middle of the night and sneak out. I remember my mum being incredibly distressed (I was about 4 at the time) and couldn't understand why nan was crying and pleading for her to unlock the door and let her go home... I really wouldn't wish that on dd - anyway, having had her so late in life and with our family's history of heart disease/stroke, I'll probably be long gone before that eventuality comes along!