Do presidents age faster in office?

At 50, which he turned yesterday, his hair is a little grayer, and his face and neck are a little more creased, than on the day he took office in January 2009.

You'd expect some of that -- he is older. But Obama, like his predecessors, has had plenty to do. He's waged battles over economic stimulus, health care and the debt ceiling. He's juggled wars in Iraq and Afghanistan; he committed to a campaign over Libya's skies; he chased Osama bin Laden; and he dealt with a massive oil disaster off the Gulf Coast.