So you don't actually have any empirical evidence for term limits doing what you say they will, and your dead set on judging people by their age and not their actual capabilities.

You're reducing a person to a number and ignoring their actual personhood. If you think that someone is mentally unfit for office, argue based on them, not their age.
Maybe it will make more sense if we replace one characteristic of a person that isn't relevant to the job with another:

The vast majority of the Democratic's higher leadership are Jewish. I like Bernie, but it is clear that Republicans, despite being shit, actually have representation of their Christian cohort and benefit from that vitality.

Religion has as much to do with the job as age.

It typically isn't the older rulers who change the fate of nations in a helpful way. Their idleness, is typically the cause of famine, decay, and disaster, because they simply didn't care or couldn't understand.

Citation needed. Hitler took power at 43. Stalin at 46. Mao at 56. Pol pot at 50. Czar Nicholas II was 50 when he died and took office at 27. Lincoln was 56. Ghandi was most influential in his 70s. FDR was in his 50s. Pretty good spread for leaders I could think of off the top of my head who were notably good or bad. Who were you thinking? "Look at history books" isn't evidence when history books are full of people of all ages doing basically everything.

https://www.thelancet.com/journals/lanhl/article/PIIS2666-7568(24)00140-5/fulltext

the majority of individuals aged 65 years and older are cognitively unimpaired. Age does not equate to cognitive decline and is not a good marker for an individual’s ability to serve in a political role. Although poor performance in younger people might be perceived as a one off, for example due to ill preparation or a poor night’s sleep, in older adults the assumption is all too often that it is due to cognitive decline.

Have you considered that that's just how your parent is, and their age has nothing to do with it?
I had aging parents too. One was basically fine until the end, other than arthritis and blood pressure. The other has a host of ailments, and the only mental change was that he got more progressive, since he had more time to read, and he refused to accept that his sense of taste was getting weaker.

You do understand that people are voting for these politicians you want to bar from office, right? Not liking who people choose to represent them is usually best acted upon by voting for someone else, or encouraging someone you like more to run, not by disqualifying the person most people preferred.

You should take a step back and consider why you hold your beliefs. Are they based on feelings about how you think the world is, or should be, or are they based in things that are objectively true? How do you know they're true?