 President Biden is the oldest man ever to hold the office of president. You know, leader of the free world and all that. In fact, he was older the day he was sworn in than President Ronald Reagan was the day that he left after eight years as president. But a growing majority of Americans believe he may not be mentally up to the job. Sworn in at the age of 78, Joe Biden is now advancing in years and says he has no idea why so many people would question his mental fitness.