There is a Jellyfish whose biology allows it to go from a fully mature Medusa, to its polyp stage and back again indefinitely.
The Turritopsis nutricula is a small jellyfish with an ability so far unique in the animal kingdom. It is the only known jellyfish to have developed the ability to return to a polyp state. Or basically, reverse its life cycle, then grow up again. This process allows the jellyfish to bypass death. In fact, there may be no natural limit to its life span.
The key lies in a process called transdifferentiation, where one type of cell is transformed into another type of cell. Some animals can undergo limited transdifferentiation and regenerate organs, such as salamanders, which can regrow limbs. Turritopsi nutricula, on the other hand, can regenerate its entire body over and over again.
There are many natural dangers to these animals, so they can still easily die. Science also doesn't have a way of knowing how old one is, so there are no records of their old age. To read about the oldest known vertebrate see my previous post Here.