Can’t vote for the first 18 years of your life? Can’t vote for the last 18 either (based on average life expectancy), just watch how things improve dramatically if such a measure is put in place.
Same logic with holding elected positions, if people younger than X can’t be elected then people older than (life expectancy - X) can’t either. And yes, I realize that it leaves very few years for men to become president in the US’ case and that’s not a bad thing at all.