It looks almost certain that John McCain will be the Republican nominee for president in November. If he becomes president then he will be the oldest person to ever become President for the first time. The argument that McCain and his supporters make is that age is irrelevant. There are certainly successful older leaders such as West Germany's Konrad Adeneaur who was Chancellor until he was 87 and the leader of his party until he was 90 and is generally considered among the best leaders of continental Europe in the 20th century. Other examples of older leaders include Vajpayee of India, Gladstone, Reagan and Sharon.
All of the men on that list were extremely successful in power, but the last two- Sharon and Reagan illustrate the dangers. Poor health, Ariel Sharon was the best Israeli PM of my lifetime but when his health failed him the leadership vacuum was filled by the lackluster Ehud Olmert whose handling of the Lebanon war was inept. Likewise Reagan is one of the most successful Presidents of all time but there has been informed speculation that by the end of his second term he was may have been suffering from the early stages of Alzheimer's. Unlike Sharon his second in command, George H.W. Bush, was competent and would have been capable of papering over the cracks if that needed doing. That is probably the most important lesson when voting for old men, take a good look at the person most likely to succeed if the worst comes to the worst.
Mind you the Americans' understandable enthusiasm for shooting their Presidents means that there is almost a 10% chance that they will be assassinated no matter how healthy they are, so taking a good look at the potential Vice Presidents is pretty important in any case.