I'd imagine it's merely a % each time they age. Like, for example, once they reach 55 a 1% chance per turn, at 60 a 30% chance each turn, so on and so on.

I'd like to know the specific numbers too though. I know that if you continually save and then reload if they die their age eventually resets. Not sure if EB fixed it so like after a certain age there's a 100% chance they'll die even if you save/load, but i know for fact this used to be what happened.