What I've been doing is giving them the +120 years trait. After like 200 years of living, you can assume they just wear away instead of "dying". But un-killable leaders maybe TOO OP even for robots lol

In that time frame, a particular leader may become obsolete.

I am currently playing with an immortal trait. I don't consider it that op to be honest (maybe I'm just not that good at the game^^)... I mean admirals and researchers can still die, but not from aging.
If you make it a species trait... people can choose for themselves. Maybe it''s possible to set it to only be active for player. At least I know there is such a function for ai empires.

E.g.: Only ai can be a fallen empire, so the condition for this is "ai only"

I think that instead of dying due to old age, having critical parts wear out, or just becoming obsolete that a robot/synthetic species would continually upgrade and repair themselves, otherwise why don't the populations die out the same as the leaders after so many years in game?

For organic races that's because the populations are assumed to have a birth rate greater than the mortality rate, so once a pop is 'full' it'll be maintained by new births and the children coming of age and entering the work force. Considering that the game is played on a scale where the standard unit of time is one day (one second on normal speed), old age wouldn't be a problem for regular populations.

I think the synthetic trait should either have an increased life span to make them longer lived in general or an extremely increased life span so that they will be highly unlikely to die of old age even in a very long game.

I agree with both Exodus and Deagroth but why not go the middle ground? Make them immortal but only if you pay the upgrade / repair fee in influence every x years, where x years depends on natural lifespawn + trait.

They don't die, they wear out - they've consistantly stopped functioning at around 75 years, and I think that's perfectly fine.

That's what I have. I also use extended trait mod. This mod is a little too buggy atm, so I assume they are Cyborgs, w/ science decetorate, that use robots to even fight their wars.

After having been hit with an Emperor who had a disastrously negative trait (-20% fleet$%?!) and having absolutely no way of deposing him, I think if the robot leaders are going to be immortal they have to have some way of being killed off and replaced, just in case their traits aren't that great.

Otherwise if they just lived in excess of 150-200 years that would work as well, but it would be a loooong wait for more positive traits.

With the extended trait mod you can research much longer lives.

Again, I think the Cyborg motief works well.

i would say put a new endtech in where they can upload their mind into a new body. but with a penelty or something.

I thought about unique building, that adds 100 years for synth forms of live empirewide (in normal game, too, if you're a fun of leader synths policy), but requires living metal (fallen empire's ring world tech).

I think true immortal rulers would be an extreme penalty for Autocratic empires: the main benefit of an Autocracy in Stellaris is the ability to build a ruler-specific structure, and having only one as opposed to another every 50 years would give you essentially no way to compete with the Agenda mechanic of Oligarchies or the ease with which Democracies can change their ruler if the need arises.
Machines would probably be continually repaired and constantly backed up, so they would be unlikely to die by consequence, but I think that an AI empire would allow people to die in order for social change to occur.
I agree with DeMordrey; an AI empire could probably make an exception for their ruler if it was necessary, but doing it repeatedly would upset the people if you didn't have enough Influence.

I like Jie's point of view. I do think that synths should last longer than organics, but having them immortal would suck big time in case of a bad ruler. Of course, you could always "console" the ruler out, but doing it through a selected event would be better. :)

Yeah i think they should be if that doesnt damage the balance too much. Or maybe they should get 250-500 years additional Lifetime. I mean an intelligent Synthetic dont really need to worry about wearing out, but you could still simulate some case of random severe disfunction by giving a very long, but limited Lifespan.
Plus that is the halfwaymeet for those who think they dont die and those who think they shouldnt get older than humans.

I fully expected my robots to last for ever... I was mortified when one died! It does not impact on the game much... I normally always have one in training... so making them immortal would have minimal game impact & just allows for "planned economies".