Fossilization is the process whereby plant and animal remains are converted from a decaying form to a preserved form. Sharks do not really have bones (only cartilage) so the part of their body that fossilizes the most readily are their teeth. Sharks are constantly losing and replacing their teeth. The lost teeth sit on the bottom of the ocean and eventually become fossilized. The organic material in their teeth gets replaced over time with different minerals from the ocean water and becomes a fossil. Some get trapped in sediments on the ocean floor and become trapped in rock, others eventually make their way via wave action to the shore.
Like any fossil, shark teeth can vary widely in age depending on when the shark lost the tooth. In general, fossils are remains that are considered to be at least 10,000 years old. Most shark teeth are several million years old, and they can date back to several hundred million years old.
When sharks loss their teeth, the teeth are sink at the bottom of the ocean and becomes buried in sediment, it begins to fossilize. The minerals in the sediment gradually replace the original tooth material. This process takes thousands of years to complete. A general rule is that a fossil is over 10000 years old.