The jellyfish turritopsis nutricula has the ability to revert from a state of sexual maturity to the immature polyp stage.  It is the only known example of a creature capable of reaching sexual maturity then returning to being sexually immature.  Thus, this jellyfish has been deemed immortal.  However, it is impossible to estimate the age of a specimen because of this ability to cycle through life stages.

About every week, SmartestYear does a post that proves that I, too, can revert to sexual immaturity.