That's the real uncertainty in any decision like this. How long will you live? The other way to look at it is how much does she really need to live on? I'm sure she's in some form of nursing care that will run more than SS pays. She either has generous family members, other assets or gets Medicaid. At 115 she probably doesn't know which.
The object of life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane -- Marcus Aurelius