This is true in large cities and suburbia as the cats have lots of threats from cars to dogs, and worst of all, people. In a farm setting (like ehh's cat) its completely different, they hunt for a decent portion of their food which means good exercise and a 'better' diet than suburban cats that mostly eat dry food.
The threats from people and heavy traffic are reduced in a 'farm type' setting and the cats are usually smart enough to stay away from the odd vehicle they do encounter.
So in closing, yes, my cats wouldn't live to 15 if they were outside because I'm in a suburban area with large dogs and feral people, if I moved bush however and let them out, there's very little reason they shouldn't live that long. Its all about the outside environment.