(My Nan had it, but I wasn't that old and we lived quite a long way away...)
It's going to touch me at some point however. It's bound to. About 1/6 of people over 80 are affected, and I know many more than 6 people.
Some see Alzheimer's as the worst tragedy of the modern age. I am not sure I entirely agree, it's certainly one of the most painful for the victim's family—possibly worse even than having them in a persistent vegetative state, at least in that case the wreckage of the person you loved isn't still trying to talk to you.
However, to my mind dementia, horrible as it is, is a subset of the big tragedy, which is that people die. I have written about this before: the inevitability of death, how it gets a little more evitable every year, and how that in itself brings interesting, new, social problems. Those are good problems to have, however. People living too long is infinitely preferable to them not living long enough. The increase in diseases we can't yet fix: dementia, cancer, diseases of senescence in general—is the direct effect of taking out all those lesser deaths who were more vulnerable to our sorcery.
None of which makes the failure of a beloved mind any more bearable.
I have been asked why this is late onset, when early onset is even more tragic. The answer is because early onset dementia is more like a horrible disease, striking down only a subset of us; however the diseases of old age, of which dementia is the one example, get everybody who lives long enough...
Late onset fallibility
He returns from walking the dog
no longer quite your father.
It's nearly your dog.
He returns from walking the dog;
he's only been gone two days,
which admits no ready explanation.
He returns from walking the dog
with a jaunty stride
and somebody else's shoes.
He returns from walking the dog:
your mother leaves without a word--
she has been dead for five years.
He returns from walking the dog
smiling strangely to himself;
scowling at you, your brother, the front room paper.
He returns from walking the dog;
seems like he's acting younger
and looking frailer than when he left.
He returns from walking the dog;
wants to speak to your sister, oblivious
that she lives in Queensland now.
He returns from his walk
with a cat on a piece of string
and seven tins of the wrong dog food.