Even if channelers in the AoL lived to be 600 to 800, that doesn't mean that channelers in the 3rd age would. After all, we live to about 80 thee days, but there have been times in humanity's history where average lifespans were about 30. And the differences were born of medical, diet, lifestyle, sanitation, and fitness reasons. Differences that still exist between 2nd and 3rd age channelers, presumably.