Q: My parents are very old and find it too difficult to fast. Are they allowed to break their fast in Ramadan?
A: Yes. If your parents are very old and find it VERY difficult to fast, they can break their fast and feed a poor person for every day they miss as a replacement. The amount of food required to give is equivalent to the kafarah/expiation ( 1 mudd of wheat or half saa’ of other staple food types).
The ruling for someone afflicted with a chronic/terminal illness is the same. If they are applicable to follow a concession such as travel, they are not obliged to feed a poor person for that day(s).
Source: Sharh al-Muntaha