UTIs can cause delirium in elderly people. They don’t cause long term or permanent symptoms of full blown dementia. People on MN are quick to suggest a UTI in an older person who is behaving strangely, but UTIs do not explain most of the cases described, where the person’s behaviour, manner of speech, personality etc have declined over time and then suddenly undergone a step change. It could definitely explain sudden delusions or hallucinations.
In Biden’s case it seems most likely that his verbal demeanour is caused by age related cognitive decline, and his stiff movements and posture are due to his severe arthritis (as per his medical report which has been linked in this thread).
I do wish there was an upper limit on the age for presidency and any high office in all countries. It’s a gruelling, stressful job that needs mental sharpness, and although many people over 80 can live well and independently and sometimes still go out to work, not many are capable of a full-on job that needs you to be in top condition mentally and physically. Trump isn’t and Biden isn’t. If someone is elected at an advanced age they should step down and allow the VP to take over for the rest of the term, if their health declines badly.
But the US seems extremely resistant to electing someone who is younger, female, black (Obama being the exception), or in any other way not an older white man.