If this old person is sane and has discernment, but he is unable to fast; then he has to feed a poor person every day. However, if he has no discernment and has begun to be senile, then neither fasting nor prayer is incumbent upon this person; because he has become like a child and he does not possess (full) comprehension/discernment. And (there is) the hadeeth: ”The pen is lifted from three”, therefore he is not obligated; rather if he fasted and prayed, his fasting and prayer is not correct, as long as he is not held accountable (by Islamic law) and is senile, and his (full) comprehension/discernment has ceased.
However, if he has discernment, is aware of what he says and he understands, and he desires to fast, but is unable; then this person is to feed a poor person every day.
[For further details, see: Al-Fataawaa Wad-Duroos Fil-Masjidil Haraam of Shaikh Abdullaah Bin Muhammad Bin Humaid (rahimahullaah): Page: 483]