One perhaps a little silly approach to mortality would be to have a stat called "health". It would be something that started out at 100 and various incidents during the harsh life of a noble would gradually drop it until at 0 your character would die. Age would factor into it but would not automatically drop it. It could be a soft thing in a way that getting wounded _might_ drop it one point at an earlier age, but later with an increased chance. Perhaps even so (unrealistically) that multiple stabs soon one after one would have a lowered chance to drop it.
So you would know where you character is at any given stage. You would know when it was "soon his time", so you could plan your RP:s around it and build his story to a closure. Again, not realistic, but storywise better. In this approach age would be a numeric representation of, well, your character's age, while health would be what you lose when you are tortured and wounded, which at the end effects the hours at your disposal.
This would turn infiltrator attacks from a nuisance to something serious, but yet not overpowered. Each stab would be a meaningful step towards the death of the target character.
100 health would be a nice round number, but it could be any other number too. 100 would be quite sufficient for creating a good storyline.
