While no creature can live forever, the jellyfish does have a certain ability to not only stop the aging process, but to actually reverse it. Jellyfish are a form of plankton, and do not move themselves around, but rather drift with ocean currents. It can't outrun predators, and it's a food source for hundreds of ocean species. Add to that other environmental stressors like disease or bacteria and it would seem to be the kind of organism that doesn't live very long. Actually, it's a little bit the opposite of that.
A jellyfish will actually throw its cellular life cycle into reverse when faced with a threat. The tentacles will drop off, it will shrink in size and once again attach itself to something in the environment, as it did when it matured and reached the polyp stage of cell development. From here, once the threat is gone, the cells begin to replicate themselves and it once again goes forward in developing. This back and forth cellular process, much like rewinding and replaying something on television could continue indefinitely, in theory, but usually does not. The process is called transdifferentiation where cells at a mature level actually change what type of cells they are, in the case of the jellyfish reverting to younger cells as they were earlier in the life cycle.