As an example in the world, I know that an example of a pure-line human, the Venerable Cal, is into his third century. He isn't majoritorily (probably not a word
) meched out, he hasn't spent copious amounts of time in the Warp, cheating the clock, and he's not a psyker, that I know of, so he doesn't have the usual Eldar/Jedi excuse of "strange energies" maintaining him longer than average, other than the Emperor's, if you listen to him. While he is a withered ancient, and supported by his chair, he is still human, really old, and cognizant. money, resources, medical treatments, and yes, perhaps a bit of the Emperor's blessings have kept him alive, "active", and mentally capable, where other men or women would be drooling basket cases in a third of the time. How much longer will he live? I don't have a clue. If he runs out of R'as alGhul's lazarus pits, or the Imperium's equivalent rejuv treatments, he might finally succumb to old age, and it's deterioration.
I'm sort of surprised I'm getting into this part of the chat, but i do believe you do sort of have a clock, and when it chimes, you are done. You have cells, and they can only divide so many times, unless they become cancerous, which is a different issue. Once you run out of divides, you can't replace damaged tissue, blood cells, or what have you, and you die. You might not refer to that as "dying of old age", and several of the conditions caused likely have a nice, medical-sounding name, but we are mortal, and given enough time, we will just stop, whether the capacitors in our heart fizzle out, our blood becomes to thin to carry oxygen, our liver and kidneys fail to scrub that blood, or what have you. Not sure why the Eldar get to cheat, and live for millennia, but they were augmented/manufactured by hyper-intelligent lizard people, so I guess that's an out for them. They do make nice crystal trees, however.
Edited by venkelos