This Diamondback Rattlesnake's rattle is unbroken (button is present)
A Rattlesnake cannot be aged simply by counting the number of rattles on its tail. The tip of the tail of a new born Rattlesnake ends in a smooth rounded, slightly pear-shaped, "button," which is the first segment of the future rattle. As the young snake grows it sheds its skin, usually several times a year. Each shed skin adds a new, loosely overlapped and interlocked segment to the rattle. Shedding twice a year will add two segments to the rattle. Shedding three times a year will add three segments to the rattle. The more a Rattlesnake sheds, the more segments are added to its rattle. When the Rattlesnake vibrates its tail, the segments click together to produce the buzzing rattle sound.
The smooth pear-shaped button of new born Rattlesnake
An unbroken rattle with a button at its tip
A broken rattle ending in a tiered nubbin
The tissue that makes up the rattle is rather like a thin brittle fingernail. Just as a human can tear a fingernail, when the rattle gets longer than about 8-10 segments, some may break off and be lost. If the string of rattle segments ends in a smooth rounded button, all the segments are there and the rattle is complete. If the rattle string ends in a squarish or tiered nubbin, the rattle has been broken and some segments have been lost.
These two factors, adding segments several times a year and losing segments through breakage, make it impossible to tell a Rattlesnake's age by simply counting its rattles.