Putting politics aside … Why don’t they have an age limit on the president (or any elected office)? E.g., you must be younger than the average life expectancy (currently men: 73yo / female: 79) - while in office. https://www.ssa.gov/oact/STATS/table4c6.html
And as others mentioned, this is a bipartisan stance, so it's not a matter of political leanings. Old people represent most of our voter-base and thus most of our represenatives.