Many ask this question, and in writing the profiles I attempt to ascertain reliable information on expected lifespans. I think this is important, because purchasing a fish that may easily live for 10, 20 or more years is a commitment. Most of us (I hope) would not buy a dog or cat (with lifespans normally of 12+ years) expecting to get rid of it in 2-3 years, and any living creature deserves as much.
Of course, an animal's lifespan in captivity may often exceed what it would normally have in the wild. Protection from disease, better nutrition, no environmental or ecological upheavals, etc. all contribute to this fact. In 1984 I acquired a frog, one of what are commonly called the Red Bellied Toad, and this species has an expected lifespan of 11-13 years according to two scientific authorities. My little fellow ended his life in his 20th year. He shared a terrarium with my last surviving newt, one of those Japanese Firebellied Newts. I bought two of them in 1983 and they happened to be a pair and bred. The last surviving offspring, born in the Spring of 1985, died in December 2006, in his 21st year; unfortunately, he might still be here today, had he not decided to climb up the glass and venture into the next room where I couldn't find him--until while removing the artificial Christmas tree I found his dried corpse.
The linked article from the excellent British magazine Practical Fishkeeping sets out some amazing lifespans. The record is held by a Japanese Koi that lived for 226 years--no, that is not a typo, it lived for more than two centuries.
This is one reason I write so frequently on providing the proper environment for our fish, be it water parameters and stability, reduced light, dark substrates, plants, wood, rocks, minimal filter flow--whatever, since all of these contribute to the fish's chances of living out its expected lifespan.
Enjoy the article. Byron. How long will my fish live? | Blog | Practical Fishkeeping