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
In a democracy we let the pleb decide if they want a 81 year old as president (or a 18 year old). It works as intended. Putting age limits on fixed-term elected officials would go against the spirit of free elections.