Most people will say 15-25 years. Some say 20-40. There was someone on here the other day (can't find the post) talking about a 72 year old woman she met who's had the same res since she was 5...that'd be a 67 year old turt
The things that come to those who wait may be the things that were left by those who got there first - Steven Tyler
I was doing a search on 'life spans' and ran accross a vet's site with the life expectancies of different animals, and according to him, RES live up to 7 years, check it out:
http://www.sonic.net/~petdoc/lifespan.htm
I found this quote from Melissa Kaplan's site:
If maintained at the proper temperatures, fed a healthy varied diet and kept in a stress-free active environment, your turtle may outlive you: some individuals have lived more than 100 years.
They key is that RES's can live for 15, 20, 30 years. Reality is that the average RES probably only lives between 3 months to a year the way most people take care of 'em.
Yes, hatchlings have something like a 90% mortality rate. But for the ones that make it to adulthood, for the captive turtle the enemy is mainly a too-high protein diet, which will cause the kidneys to give out between 15-20 years, and for the wild ones the enemy is humans (distruction of habitats, turtle trade, etc.).
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-