Because C-14 dating uses the decay rate of Carbon 14 as its measure (more on this later), this gives upper and lower limits to the dating method. If the sample is too young, not enough decay has occurred. This means that the upper limit of C-14 dating is currently around 50,000 years ago.

Why are there limits to how far back in time C 14 can be useful?

Because of the short length of the carbon-14 half-life, carbon dating is only accurate for items that are thousands to tens of thousands of years old. Most rocks of interest are much older than this. Geologists must therefore use elements with longer half-lives.