When most animals reach the end of their life span, they start to break down and exhibit senescence, which we know as aging: grey hair, tiredness, reduced appetite. Some animals, like crocodiles or tortoises, exhibit negligible senescence, or don't age - which means at 50 or 100 they're as perfectly fit as ever. As long as they have enough food, they can just keep growing and growing. Of course, this doesn't make them immortal, as they can still fall prey to accidents or disease. But they stay fairly biologically young - which is why we get spry 112-year-old crocs and 183-year-old tortoises.
Owls keep pets.