I think Stanley Ketchel was shot at quite an early age. But ,through a kitchen window, rather than over the hill.
And they are also matched more carefully today (especially much less often) and advances in health&medical science makes them overcome injuries quicker and also train in ways that help in avoiding injuries etc. I'd say you have a similar thing going in most big sports today. Athletes that reallly take care of themselves and make the most of the knowledge avalaible when it comes to nutrition, training, life-style etc can stay active at a high level for a long time. Just look at guys like Maldini, Giggs and Zanetti in football for example.
How about the 1950's when old fighters like Walcott, Robinson, Moore, and a few others were the top of the sport. Oh yeah and Old Bones Joe Brown.