The data is supposed to be at the Adminstration on Aging's site, but I couldn't get some of the PDF files to download.
http://www.aoa.gov/prof/Statistics/statistics.asp
Still, I found a few other web sites that quoted figures of "average age of widowhood" at 55 to 58 years old.
I think the reason this is so early is that:
1) Men die more frequently than women of accidents/homicides/combat injuries, etc at early ages.
2) If a couple lives long enough, they are out of the window where homicides and vehicle accidents are big factors. After that point, it is increasingly likely that the man will outlive the woman (though it is still more likely she'll outlive him)--if that happens, she doesn't become a widow and those statistics don't get entered into the average for widowhood.
3) Finally, I'd guess that the male is likey to be older than the female in most couples, which just exacerbates the already existing difference life expectancy (since we're talking "average age" rather than "median age" of widowhood, cases where a rich guy gets "trophy wife" 30 years younger and dies the next week offset lots of cases where couples of more similar age grow old together.)
I think if hubby isn't racing stock cars, isn't robing liquor stores, and isn't looking for a youger replacement bride, then you're likely to grow old together. If he
does bring home a young thing, the Mrs can shoot him before the divorce and do her part to help increase the overall "age of widowhood" statistic.