Average life expectancy doesn't accurately show when people would die from old age. With so many people dying due to violence in those days the average gets dragged down. I think the most ideal way of figuring that out (if possible) would be to look at the life expectancy of priest and nuns since they were typically protected from violence.
It's difficult to find good sources for medieval life expectancy, since you really need to know what are the assumptions. We should completely discount child mortality, for example, because the characters start at 16. I found an interesting one that says that in the 13th century, members of the royal family of Wales (which were of similar social classes as most nobles in BM) that reached adulthood had an average and median life expectancy of 49:
http://www.sarahwoodbury.com/life-expectancy-in-the-middle-ages/
This include those who were killed in warfare and women who died in childbirth, but even if you discount those, the average life expectancy is still 49.
Also, from the same source:
“Anglo-Saxons back in the Early Middle Ages (400 to 1000 A.D.) lived short lives and were buried in cemeteries, much like Englishmen today. Field workers unearthed 65 burials (400 to 1000 A.D.) from Anglo-Saxon cemeteries in England and found none who lived past 45.
Kings did better. The mean life expectancy of kings of Scotland and England, reigning from 1000 A.D. to 1600 A.D. were 51 and 48 years, respectively. Their monks did not fare as well. In the Carmelite Abbey, only five percent survived past 45.”
