The folktale that claims that a rattlesnake' s age can be determined from the number of sections in its rattle is false, but only because the rattles are brittle and sometimes partially or completely break off. So if they were not so brittle, one could reliably determine a rattlesnake' s age simply from the number of sections in its rattle, because one new section is formed each time a rattlesnake molts.
Rattlesnakes molt exactly once a year is an assumption the argument requires in order for its conclusion to be properly drawn.