[dragonwriter]

> That's a great question, my understanding was that if one survived birth (infant mortality skews average lifespan quite a bit)

Infant mortality does, but that's more than just surviving birth, but more like the first five years to get past the point where it skews results.

> From the sound of it, the knights certainly weren't going hungry

Well, sure, and you wouldn't expect knights—a narrow elite class—to be goings hungry even if food insecurity was common for the masses. But they probably lived longer mostly because the average expectancy for people who had survived to the age at which you become a knight of the order was much greater than the average population-wide life expectancy, because the large proportion that died very young did so earlier.