You can tell how old a rattlesnake is by counting the sections on its rattle.
When a rattlesnake is born, it already has first section of its rattle. Every time the snake sheds its skin, a new section is added. A rattler sheds its skin any time it needs to - not on its birthday. And sometimes sections of the rattle break off. So, not much you can learn by counting the sections of a snake's rattle.