Question: What is the age that if reached by old people makes it permissible for them not to observe Sawm (Fast) of the blessed month of Ramadaan and to feed the poor instead? How should they feed the poor, i.e., should they do so at the end of Ramadaan or day by day?
Answer: There is no fixed age at which old people are permitted not to observe Sawm. Rather, it depends on the permanent inability to observe Sawm. To put it in plain words, if one cannot endure Sawm due to being old, it is permissible for him not to fast and to feed a poor person (as expiation) for every day (wherein he breaks Sawm) half a Saa` (1 Saa`=3 kg. Approx.) of the common food of the country. It is also permissible for him to give out all the fifteen Saa`s at the beginning of Ramadaan… read more here.