Grandma Flanders must be pretty old (and possibly dead?) but considering Ned's own advanced age (possibly) she must be very old indeed. I can't find any direct evidence for anyone's age, except for Ned saying he's 60 and having living parents.
As seen in the clip below, she states she has just turned 100.
NED: Now, sir, don't you waffle on that brunch. Grandma Flanders can watch the kids.
GRANDMA FLANDERS: Hello, Joe.
HOMER: Doesn't know who I am... has trouble moving... I'd say we have ourselves babysitter.
GRANDMA FLANDERS: I just turned a hundred.
Although she uses the wrong name, Ned doesn't correct this or the age she gives. It's possible that the age is wrong but, unless the character appears again with a definitive birth and/or death date, we may not know for certain.