I have been curious about this also. With the explosion of herpetoculture in the past 20 or so years, seems like lots of keepers have snakes that are much older than I thought possible many years ago. Read somewhere recently about a timber rattlesnake that was in its 50's when it died. The old longevity records are largely from zoos, but I believe some private herpetoculturists take better care of their snakes than zoos do. I think FR mentioned he has been observing a group of gilas for maybe 30 years.