The Constitution is quite clear that to be President of the United States, an individual must fulfill only three conditions: be a natural born citizen, be at least thirty five years old, and reside within the United States for at least fourteen years. Thankfully, there are no Constitutional requirements based on race, ethnicity, sex, sexual orientation, religion, physical infirmities, or any of the other personal attributes that are made to seem so important today. But while the founding fathers recognized that an individual seeking to be President should have a minimum amount of life experience, they did not account for the health effects of aging and made no provisions for an upper age limit.
Most people probably know that Theodore Roosevelt was the youngest president at 42 and Ronald Reagan was the oldest at 69. The average (and median) age of presidents at inauguration is about 55, shown in the histogram below. The average (and median) age of presidents at death is about 70, making Reagan's election at 69 curious and his reelection at 73 baffling. At the time Reagan was first elected, he was older than all of the previous presidents at the end of their terms except one (Eisenhower was a year older), and older than half of the previous president were when they died. Some historians believe Reagan suffered from dementia, especially in his second term, long before he was diagnosed with Alzheimer's disease. Presidents, on average, live about ten years after the end of their terms, but Reagan beat the average living 16 years after his term to an age of 93.
A recent study by S. Jay Olshansky, a professor of public health at the University of Illinois at Chicago, found that about two-thirds of Presidents lived an average of five years longer than their contemporaries. Olshansky characterized the longevity of the presidents as a classic illustration of the benefits of socioeconomic status, in that the differences were most likely attributable to the privilege of wealth, education and medical access. All but ten of the presidents were college-educated, they were all wealthy, and they all had access to medical care.
Some people argue that an upper age limit should be decided on an individual basis by the electorate. The same argument is also applied to the lower age limit of thirty-five years, but the Constitution cuts off that debate. The problem with that argument for an upper age limit is that the electorate usually doesn't get the kind of information that would allow them to make an informed decision.
A good many of America's Presidents have had medical issues, some age related and some not. Washington wore wooden dentures after he lost all of his teeth to infection. F.D. Roosevelt had polio, Arthur had Bright's disease, Kennedy had Addison's disease, and G.H.W. Bush had Graves' disease. Eisenhower had a heart attack and a stroke while in office. Cleveland and Reagan had cancers that required surgery. Wilson had a stroke that incapacitated him. W.H. Harrison died of "bilious pleurisy" and Z. Taylor died of gastroenteritis, both while in office. Short of a death in office, these maladies were and are largely kept secret or downplayed in the media.
Consider the following figure illustrating each President's age at birth, age at death (or current age), and term in office. Compare that information to the bottom of the chart where the leading Republican candidates are listed, centered over the age they will be in January 2012. Compared to Reagan's age at inauguration, Paul would be 7 years older and Gingrich would be about the same age. In 2008, John McCain was 3 years older than Reagan when he ran in 2008.
In 2016, The Democrats may face the same challenge. Of the candidates who ran or were considered as potential candidates in 2008, most are older than Reagan was in 1980. Fresh faces may need to emerge.
There is no upper limit on age for a candidate for president, so voters will have to decide for themselves how old is too old.