Not getting into any other parts of your post but this point is one that is generally misunderstood.
Short Average Life Expectancy numbers are driven mainly by high infant mortality. Adults commonly lived much longer than 30. 1000 years ago we weren't freaking Neanderthals. (Just setting the record straight.)
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1000 years ago there was no penicillin, no quinine, no way to fix appendicitis, ulcers, tonsillitis etc with simple surgery.
More people died with Pneumonia than any other cause. It made lot of sense to get girls married early when they were able to conceive.