Here are a couple of general questions: How much correlation is there between the age somebody looks and their mental age? Romney looks much younger than 65, but how much can you judge a book by its cover?
How much does the Mormon lifestyle of no smoking, no drinking, and no caffeine matter?
By the way, the presumed frontrunner for the Democrats in 2016 is Hillary, who is about the same age as Romney. (Biden is even older.) The Democrats, however, have been more fad-driven and less likely to reward party warhorses who wait their turn.