Well, by natural causes I did not mean diseases, but rather failure and breakdown of the body as it gets worn out. This would probably happen EVENTUALLY but the during 100,000 years we have seen, beings like Takua, Herlyx, Orde, Karzahni, Velika, Tren Krom, Artakha and Shadow Stealer do not seem to show any such signs. But then again, there is regeneration and, their entire body could probably be replaced, but then again, every part could pretty much be replaced in a human, so, I'd go with Helryx and Orde as the prime example, since, as far as we know, they've not been dealing with any anatomical failures, and as far as we know, have not transformed. Though we have not seen how they look, I do not believe they show any signs. Essentially, most MU beings usually get transformed undoing many of the effects of ageing, but also, it's really slow, since beings who have been alive 100,000 years are still fine.
Side note; there is the Kanohi of ageing, aswell as Voporak's power, so yeah. There would age, but just very slow. Then again, doing some math here...
Based on this http://www.bzpower.com/board/blog/59/entry-111284-farshtey-feed-final-entry/ Raanu would be about 60 human years, and that Raanu is 265,000 years old, since he was born little before Lein's death. So, since we want to know 100, you can cover that into being 100%, this would make 60 years be 60% or 0,6. So, what we do is take 265,000/0,6 which will give us 100%, being 416,666,666... years. So about 400-450 thousand years would be equivalent to 100 years of age for a human when it comes to Agori, and probably something close to that among Bone Hunters, Agori and Skrall.
So yeah. Maybe they age at the same rate as Agori? They'd only be around 25 or so human years.
(note, Agori would probably grow up at a faster rate than that, rather than be babies for thousands of years, which seems kind of implausable when taking Kiina into account, even if teenagers were allowed to fight in the Core War. Iirc from the LMBs, though I may be wrong, Gelu is also of a similair age, and both of them participated in the Core War.)
Minor note, Greg's example of 2,000 years, man that's such a small number in BIONICLE