Hi Anne , thank you for your detailed reply. The issue of "average" life span is a thorny one. so called averages can be skewed by a number of things one of which is that the upper classes had a longer life expectancy, hence Elinor of Aquataine living to age 82.
Also, age , like height fluctuated greatly over a thousand years. The Anglo Saxons were much smaller than the Romans who invaded Britain,however, within a generation or two, they had grown in stature. The Romans introduced new foods and more efficient farming methods. And of course there were great fluctuations in climate over the years, this too affected nutrition related health.
I have heard the "sleeping sitting up story" as a reason for short beds. But what would explain short tombs, low doorways, and low cielings. ?
The Armour of Henry V111 survived because there was so much of it. He had many suits of armour made over the years. He even imported Dutch armour makers into England, they set up shop in Greenwich and were constantly employed by Henry.
   I don't think that Historians tend to generalize on the basis of very little knowledge.. We know a great deal about travel in the Middle Ages..who did it , why and how they did it. The vast majority of people did not venture far because they had not the means to do so.
On what basis does historical evidence get revised ?
I have really enjoyed your observations and would enjoy discussing it further, however , this thread is likely not the place to do so.
   Thanks again