turritopsis nutricula, a species which can “[cycle from a mature adult stage to an immature polyp stage and back again][link],” may have no natural limit on it’s lifespan.
Granted, it can still get beaten to death or otherwise killed through violence or stupidity. Just not old age.
[link]: http://green.yahoo.com/blog/guest_bloggers/26/the-world-s-only-immortal-animal.html “The world’s only immortal animal | Yahoo! Green”