About what would be the average life expectancy in the middle ages? The show says Tywin is 67, and he seems in good health (he’s leading an army, no gout or difficulties riding). Is that normal?

Well, I’ve done some estimates. 67 would be beyond the average, as would be Walder Frey’s 92 years, but you did get some people in the Middle Ages who defied the trends, and of course it helps to be of the nobility where you don’t run into the problems with malnutrition that increased child mortality so much, and you’re engaged in less heavy manual labor which ages your body a lot.   

As for Tywin, the show exaggerates his age somewhat. Tywin was only 58 at the time of his death.