I'm not so sure. My wife has ran a marathon once, and she was able to train for it in just 6 months. It has more to do with determination, hard-work, and mental focus/toughness than atheltic ability.
Regardless of age, if he took care of himself and was already in decent health, I don't doubt it. He might not have trained for a marathon prior to that age, but might have done other things to keep in shape.
Another reason I tend to believe is that it was NOT some chain letter/junk sent via email. It was reported by many reputable new sources worldwide (try Googling it!).