It is definitely troubling, worrisome and tragic-horrible that there's nothing stopping some centenarian from becoming President.
Even more frightening is the fact that it's not impossible for this to happen. The president pro tempore of the United States Senate is third in the line of succession to the Presidency behind only the Vice President and the Speaker of the House. And since the president pro tempore is traditionally the senior Senator from the majority party, that person is usually super dooper old.
Had Jim Jeffords not switched sides, the months immediately after 911 may have felt even scarier.
Strom Thurmond was third in the Presidential line of succession when Jeffords did his switch and he won 39 Electoral votes as a pro-segregation candidate in the 1948 Presidential election against Harry S Truman.
President Strom in '02-'03 *and* with the pressure to "deal with" Saddam Hussein?
I doubt Thurmond would have allowed himself to become President at age 100. I doubt that Chuck Grassley would accept the Presidency at age 90-something.
Pelosi would have taken the job, and she's not up to snuff. She was once able to be President, but those days have passed.