If I remember correctly, the whole 'average lifespan' calculation for the middle ages was skewed not only by high infant mortality but by the many things that killed people in the middle ages that don't kill us now. Nowadays if they get you to the hospital in time, even if your brain dies the rest of you can probably be kept alive; in those days even simple things could kill you, and there were a lot more deadly accidents and diseases we don't get.
For instance, appendicitis would likely have been deadly-- it was probably diagnosed as a 'surfeit of...' whatever! (I have in my possession a circa 1920 herbal medicine book that says the practice of operating for appendicitis is just a fad, brought on by the appendectomy of an English Royal at the turn of the century, and that appendectomies are always unnecessary.)
I was also thinking about how often the herbal treatments for lack of menstruation that we see in our medieval books may have actually been treatments for PMS or even pallatives for Pelvic Inflammatory Disease, ovarian cysts, or other things that we don't suffer from nowadays without treatment and therefore don't think of as amenorrhea.
On the medieval side: The Tarim mummies, for those who aren't familiar
with them, are a group of natural mummies found in the Tarim basin of
China. Despite being Bronze age and being buried long before anyone
from the west was supposed to (by our knowledge) traveling that far
eastward, they appear to be of Celtic origin and the adults were over 6
ft. tall. There is evidence that they actually introduced writing and
chariots to China. So the myth that our medieval ancestors were shorter
than us is once again debunked. The myth that our medieval ancestors
lived shorter lifespans is also debunked by these and other finds. The
question is do you think it was diet?