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
Politicians who refuse to retire is a huge, bipartisan problem in the US affecting both genders. Putting an age limit on the president or members of congress would probably have to be done via a constitutional amendment, because the Supreme Court has ruled in the past that states cannot add eligibility requirements that are not in the constitution. One reform that would help in Congress would be to modify the seniority system. Right now, members of Congress gain seniority as long as they keep being re-elected, so you end up with a situation where all the most powerful committees are headed by ancient legislators teetering at death's door. Since those committee chairs are very powerful, they get lots of campaign donations which help keep them in office until God calls them home. I think modulo arithmetic should be used to calculate seniority. A member of the House should have their seniority set to their time in office modulo 10 - after five terms they are reset to seniority zero. For senators, maybe modulo 18 could be used.