Turritopsis nutricula is a hydrozoan with a life cycle in which it reverts to the polyp stage after becoming sexually mature. It is the only known case of a metazoan capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage (Piraino et al. 1996, p. 302). It does this through the cell development process of transdifferentiation. Theoretically, this cycle can repeat indefinitely, rendering it effectively immortal.
{Image: the Turritopsis nutricula‘s life cycle, which shows how it is able to live indefinitely through an endless loop of renewal. WTF!}