Reference: Fataawa Ramadhaan – Volume 1, Page 240, Fatwa No. 187;
Fiqh al-‘Ibaadaat – Page 187
Question: What if during the day while he was sane and was fasting, he then became senile?
Response: Were he to [have started the day in a state of sanity, and then later] become senile during the day, then his fast would be nullified. This is because he becomes of those from whom acts of worship are not required. Also, if [having started the day in a state of senility] he were to [later] become sane during the day, [then] he is not required to fast that day, rather it is upon him to make up [the fast] for that day. Similar is the case of the one who [starts the day in a state of sanity, and then later] becomes senile during the day, because earlier during the day [while he was sane] he was considered amongst those upon whom fasting is mandatory.