If Romney wins, he'd likely be renominated in 2016, when he will be 69, which isn't that young anymore. The Republicans had a lot of success with older candidates in the 1980s, but less since then: the elder Bush in 1992, Dole in 1996, McCain in 2008. Both Dole and McCain suffered terrible physical agony in their country's service (Dole almost died of his wounds several years after World War II), which doesn't help keep a man youthful-seeming. In contrast, Romney (like Obama) has enjoyed a physically comfortable life with no outside stresses.
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.