Reliability of carbon dating
Maybe it says something about human nature, that the only form of life we have created so far is purely destructive.
Carbon dating is somewhat accurate because we are able to determine what the ratio was in the unobservable past to a certain extent.
By taking a carboniferous specimen of known age (that is, a specimen which we are able to date with reasonable certainty through some archaeological means), scientists are able to determine what the ratio was during a specimen's lifetime.
Note that, contrary to a popular misconception, carbon dating is not used to date rocks at millions of years old.
Before we get into the details of how radiometric dating methods are used, we need to review some preliminary concepts from chemistry.
But it's not off by billions of years to give young earthers a valid argument.