My birds' vet says the average is 15-25 years. He said a great majority of the tiels that come into his office are well into their 20s. Many don't live as long, but its mainly because people are not well educated on their nutritional needs. Kind of like unhealthy foods can greatly shorten the lifespan of humans and take it well below the average. I go to a cockatiel board, and there's a member there who has a 25 year old tiel, and she says she's been feeding him only seeds his whole life.....so even with just seeds, they *can* live long. You'd be amazed at how old so many tiels on that board are! Even in the wild, according to what I've read, the average is 20-25. There have been tiels that have lived up to 40 years.........just checked on the web and found that, but that is an amazing age for a tiel. If well taken care of, they have the potential to live well into their 20s, or even longer.