I (very rapidly, so there are probably some mistakes) put together some data about the kings of France during CK's timespan.
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On average, they died at 43.4. Lots of them died of various diseases, but mostly dysentery, and it's also probably the case for a lot of the "unknowns".
It seems like generally, a king with a full life died between 50 and 70. None of them lived more than 66 years.
Only one died in battle. So I don't think it should necessarily be a more frequent cause of death - high nobility was rarely killed in combat anyway, because their ransoms were very valuable.
However, there are indeed a lot of hunting and horse accidents leading to death.
Now, of course it's just the data for kings of France. If I have the time, I'll try to make something that looks better and includes more data for various ranks of nobility (trying to respect the "count, duke, king" system of CK) in more kingdoms.