McCoys exceptionalism has, I believe been attributed to his encounter with Fabrini medical know-how. But why him and no one else hasn't been adequately explained. The oldest people now don't really live all that much longer than the oldest people did a hundred years ago, it's just that more people are "maxing out" due to improved medical technology. What also skews the averages is that we don't have as much childhood death as we used to. I suspect that by Trek times the story is much the same. Some small improvement in "maximum lifespan", much increase in "average lifespan". McCoy, well, he's just to onery to die.