There is a jellyfish that can basically hit the reset button on its own life.

The "immortal jellyfish" (Turritopsis dohrnii) can revert from its adult form back into its juvenile polyp stage when stressed or damaged, essentially restarting its life cycle. In theory, it can repeat this process indefinitely.

It can still be eaten, get sick, or otherwise die — so it's not actually invincible.

But apparently dying of old age was optional