That is true, but Motorcycles are kind of the same way. Ricky Carmichael hung up his boots at the ripe old age of 27, I think he could have gone on. Maybe he could have pulled a K-dub by only racing Motocross or Supercross?
It all depends on how well his joints hold up and I'm sure he's got a dietitian and physician working just for that purpose. Certain minerals will make joints last longer, and I'm sure he's eating foods with a LOT of those minerals.