How long do Oni live?

[oni15]

What type if life span do Oni have?

[Zhangar]

I'm pretty sure the answer to this is "until they die by violence."

They're spirits made flesh, and even the weakest of them have fast healing, while the rest have some level of regeneration.

My general rule of thumb is that anything with fast healing or regeneration can live pretty much forever, as they possess bodies that don't wear down.

Edit: That and many of the oni leaders from the Jade Regent AP are really, really old - sometimes well over a 1,000 years old, if not older (it's been years since I played through the AP at this point, so my memory can be bit fuzzy as to the fine details).

[oni15]

I thought that, I have a friend that disagrees with me on it, at the time he was the GM, I haven't played the Jade Regent AP and could never find anything online about how long they live and knowing him I would need citations.
I had forgotten the disagreement until I learned trolls are immortal until Violence occurs because regeneration.

[Creon Vizcarra]

Oni are a bit odd, because they are presented like other fiends, which is to say scary immortal antagonists.

However, they have the native subtype, which implies that they are not immortal, like tieflings and aasimar.

Rakshasa are in a similar boat. I wish I had a citation for you, but I believe Rakshasa are mortal but live for thousands of years before dying and reincarnating (except for the appropriately named demigod Rakshasa Immortals).

Oni may function similarly. The nature spirit at their core may be immortal, but the body is not. So when the body dies the spirit has to construct a new one.