The world seems surprised that an 85-year-old globe-trotting pope who just started tweeting wants to resign, but should it be? Maybe what should be surprising is that more leaders his age do not, considering the toll aging takes on bodies and minds amid a culture of constant communication and change.

There may be more behind the story of why Pope Benedict XVI decided to leave a job normally held for life. But the pontiff made it about age. He said the job called for "both strength of mind and body" and said his was deteriorating. He spoke of "today's world, subject to so many rapid changes," implying a difficulty keeping up despite his recent debut on Twitter.

"This seemed to me a very brave, courageous decision," especially because older people often don't recognize their own decline, said Dr. Seth Landefeld, an expert on aging and chairman of medicine at the University of Alabama at Birmingham.