According to the wiki:
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				Small chance for all enemies in the entire world to gain an affliction or die of old age. The chance is about 1% to die and 5% to get a random affliction. Even pretenders can be affected. For all provinces, kills 2% pop per turn and 0-3 unrest (greater in enemy dominions). Additionally, units age a random number of years every turn (between 1 and 5 (?) years).
			
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 So are enemy units effected more?