Immortal jellyfish (Turritopsis dohrnii)

This may seem like just a myth, but it's not. Turritopsis Dohrnii is found in the Mediterranean Sea and in Japanese waters. It is one of the known cases of a species being able to convert into an earlier stage of life and live life all over again. 

The conversion cycle from mature to immature

They can use this cycle when sick or old and this can go on indefinitely without disease or predators.