The immortal jellyfish - Turritopsis nutricula - can revert back to a polyp after it has reached sexual maturity. The process of growing up and ‘growing down’ can occur an unlimited number of times rendering the animal essentially immortal. And that means! - Given it doesn’t get eaten or diseased there might be some real old examples of these Benjamin Buttons floating about in the ocean. It’d be impossible to tell their age as well, given they tend to manipulate their body’s maturity on a biological level. Cool thought though.