I see this argument coming back time and time again, but it's still one I have trouble with sometimes. It seems to be somewhat the last excuse people use to judge that the Paleo diet can't be this good.
My general answer is that age of death is a median and that in the paleolithic era a lot of children died at birth or shortly thereafter and that a lot of people died of accidents, injuries or infections, thus skewing the median age of death. I go on an say that today we have emergency services and we are able to sustain sick people for a long time with no regard for their quality of life.
I also explain that people didn't die of heart attacks, diabetes, obesity... But instead stay active until the very end.
So, what's your best counter argument to this annoying excuse that we shouldn't eat like caveman since he died young?