The World's Longest Living Thing

Of course the wording of this fact could come to mean two things, do I mean as in length or age? I am referring to age in this context. Of course certain creatures beneath the sea have been known to possess a progressively long life, such as the Jellyfish, a creature theoretically possible of undergoing a regeneration process, not unlike Doctor Who, in which it can revert to a younger state and grow old again. Looking at the world of plants might appear to be a more logical answer as certain trees live to be hundreds of years old, such as the Giant Redwoods. Or you could just try to be funny and say Cher or something.
This particular organism is potentially immortal. The world's longest living thing is actually Lichen, a symbiotic organism between fungus and algae. An odd couple of a different variety but a beneficial one as each component perfectly complements each other despite being wildly different, not unlike the best sitcoms.
The fungus provides an environment that it can comfortably live in and the algae is capable of continually photosynthesizing and due to the protection of the fungus, is kept alive to carry on photosynthesizing which provides the fungus with enough energy to survive and carry on protecting it and... you understand the idea by now hopefully. 
One particular example is estimated at being 9000 years old and not only is this organism immortal, it is everywhere. For over 8% of the surface of the Earth it is the dominant vegetation. So if you're worried about a Day of the Triffids style apocalyptic event, start destroying the immortal Lichen first.