You shouldnt be in power at that age. Your body is either failing, beginning fail, or could fail within minutes.
Lets even set aside the massive mental degredation, of which the outliers are few and far far far between. Hows your heart? Your liver? Your thyroid? How many politicians have to have sneaky hidden surgeries and procedures to hide their failing bodies from the public? (Spoiler, its a lot)
These people are supposed to lead nations. Thats like trusting the lead sled dog pulling life saving supplies across the tundras of the north to the 20 year old husky who is missing an eye and a hind leg.
This isnt even about disabilities. You cannot out-fit the passage of time. Your body cant out juke aging. And we know the human limits are sputtering out by that point regardless of your natural levels of ability.
They. Dont. Need. That. Power. They need to retire, care for their health, and enjoy their final years. They dont need to hoard power like some pathetic sniveling dragon guarding a pile of gold.
The only reason we dont have that no brainer limit already is because the walking mummies arent willing to give up their thrones. Because they crave power. Which is proof they do not deserve it, and need to go.