I used to think the stress myth was true, but then I saw a picture of Al Gore recently. He looks just as bad if not worse than Bill Clinton at this point and he infamously never became President. Hillary Clinton also looks to be in pretty bad shape lately. With every passing year she starts to look more and more like Benjamin Franklin. Meanwhile George W. Bush held the office for two terms and doesn't look all that old. I don't think the stress ever got to him, most likely because he chose not to give a shit, but it wasn't simply being President that made him greyer.
Perhaps it's the effort a President puts into the job that makes him age more. That would explain why Obama went in looking like Tiger Woods and looks more like Frederick Douglass now. The bottom line is you take a picture of someone today and compare it to eight years from now it'll look drastically different no matter what. Just think about if a baby was elected President. You'd look at its picture eight years later and think "Jesus... Being President really does change people!"