Mom frequently says "I am just too old". How do I respond?

I would tell her " If you are still here, that means God has work here for you to do, so ask Him what it is. God has a plan." and "Maybe God knows I still need you here, for a little bit longer, OK?"

Is she otherwise healthy? Being 92 and live with a sister that has dementia? Oh my, no wonder she is tired! I do hope she is not the caregiver, that would be way too much for her to handle. Could you let us know what the living situation is? Are they in AL? I know at 59 being a caregiver is hard and tiring, it would be impossible for me at 92! Give her a big hug and please let us know if they are under care or going it mostly alone (with you helping, of course).

I amend my answer after re-reading the question. Please look into a memory care center for your aunt, at 92, I think mama deserves not to be a caretaker for her 88 sister who has dementia. I'm not suggesting that YOU should be the caregiver, either. You live a 100 miles away and probably have a family and that would be too much to ask. But perhaps having them in AL or progressive care facility would be the best for both of them and you!