For some reason this info is not in the wiki.
There are 4 columns in the database (models files) that apparently are not used (set to NULL):
I do not know how to use them, but Clovis (in his mod for AACW) modified it and for example in the model file of General B Bragg you can see these lines that are not in vanilla game.
CombatDeathChance = 120
DeathChance = 0
DeathChanceInc = 0
Also in CWII in the file Gamelogic.opt you can see the following:
ldrDeathBySickness = 0 // Added 111227: if = 1, leaders can die from sickness, old age, or practices better not discussed here.