It's a fact of life: we grow older. But if you're President of the United States of America, it would appear that this process happens at an accelerated rate, the most obvious example being GWB, who, as he gets ready to leave office, looks about 15, 20 years older than he did coming in. Presidents Jimmy Carter and George H.W. Bush didn't have much opportunity to age with only one term in office, but Ronald Reagan and Bill Clinton certainly did. See the before and after images below.
Just campaigning alone added some grays to Obama's head, but his barber says the President-elect is not bothered by it. See a slideshow of other 2008 campaign transformations or check out a slideshow of GWB's casual 'ready for retirement' clothes.