Reference: Fataawa Ramadhaan – Volume 1, Page 243, Fatwa No.191;
Fataawa as-Siyaam libni Jibreen – Pages 80
Question: When are the elderly not obligated to fast during Ramadhaan?
Response: Whenever an elderly person is incapable of fasting, the obligation is removed from him and takes the form of feeding [the poor and needy]. The following aayah refers to this:
So, if they reach an age wherein they have no mental ability or comprehension, then the obligation of the fast is removed, according to the correct opinion, without substitute, because of his being regarded as amongst those from whom the pen has been raised (as occurs in the hadeeth), since he is more worthy of this [obligation] being removed than the young.