If an elderly is unable to remember the Ftiha and doesn’t recognize day from night, this person is not obliged to pray as he is not considered to be in his mental health.
If he is unable to move, he must pray as mentioned in the hadeeth; standing up. if he is unable, sitting down and if he is unable, lying on his side. Prayer can’t be skipped for any reason as long as he has his mind with him. he must pray as it is possible and convenient.
Same this applies to his purity. If he is unable to clean himself or stop himself from continuously and unintentionally urinating, he has to clean himself after the athan is due and make wudhu and pray even if he is still dripping.