The C14 will undergo radioactive decay, and after 5730 years, half of it will be gone. So, if we find such a body, the amount of C14 in it will tell us how long ago it was alive. The method doesn't work on things which didn't get their carbon from the air.
This leaves out aquatic creatures, since their carbon might (for example) come from dissolved carbonate rock.
Rarely a week goes by without someone emailing me with a comment similar to the following: "They have found some fossils near where I live and have carbon-dated them to be 100 million years old".
This process is called radiometric or radioactive dating.
That causes a dating problem with any animal that eats seafood. After about ten half-lives, there's very little C14 left.
So, anything more than about 50,000 years old probably can't be dated at all.
There is little of interest in what is by far the most common and stable kind, carbon-12.
Carbon-13 is also stable, radioactively, but carbon-14 is radioactive.