Myth: You can tell a rattlesnake's age by the number of rattles it has.
Fact: Rattlesnakes are born with one "rattle", which is called a button. Each time it sheds, it gains a new "rattle", or segment. Snakes often shed more than once a year, so one rattle per year of age would not be accurate. In addition, when the string gets too long, the snake will break it off intentionally. It is theorized that they do this because there is an optimum length for better sound. I have a 23 year old rattlesnake who regularly breaks it's rattle string, and usually has 7-12 segments.
Once a snake reaches adulthood, it is virtually impossible to determine age.