Immortality isn’t as impossible as we have come to think, unfortunately, humans don’t have such an ability, however… A certain species of jellyfish (and tree) can get away with it.
The turritopsis nutricula, or immortal jellyfish for you and me, is only ~5mm in size, and has the ability to regress back to it’s infancy stage, which means that it doesn’t have to get old. The immortal jellyfish can do this as much as it likes, forever. Unfortunately these immortals can still be killed in other ways, such as being eaten, and most won’t live past the age of 10 years. Such a waste.
There is also a tree that preceded dinosaurs that is technically immortal because basically the entire forest is made up of one root system (like siamese twins sharing the same organs), and every time a new tree grows it just extends the same tree. I cannot find information about this tree though, I forgot it’s name which doesn’t help.