Is the prayer of a father valid if he prays sitting in a chair, prostrates only once, and refuses to believe whoever reminds him that there is another prostration remaining, knowing that he is elderly and suffers from memory loss?

If a person reaches old age but retains their mental faculties, prayer does not become waived for them, and they should pray according to their ability, whether standing, sitting, or lying on their side, because mental capacity is the basis of religious obligation. If they pray alone, their prayer is valid based on their certainty, and Allah will overlook any mistakes they make, given their effort. It is preferable to help them perform congregational prayers in the mosque if it is not difficult for them, so they can follow the Imam. However, if they become senile and their mind deteriorates, prayer is not obligatory for them, and they will not be held accountable for any deficiencies in their prayer.