Counterexamples to an Old Earth
It takes only one "counterexample" to disprove the theory of a Young Earth. Most of the "evidence" for an Young Earth is based on claims that lack testability, as in Biblical Literalism, and hence would not even satisfy minimum requirements for admissibility in a court of law. As with any logical proposition, one contradiction disproves the proposed rule. If each of these counterexamples has merely a 99% chance of being valid -- an underestimate -- then the probability that the Earth is billions of years old is less than 0.001%. In other words, the Earth must be old with a likelihood of greater than 99.999999999%. The same analysis can be applied to the Old Earth theory, but no valid counterexamples have been found in centuries of creationist attempts. In addition, there are over 100 pieces of evidence for an oldearth and universe.
The motivation for Creationists to insist on falsely teaching that the Earth is young is to coerce children to accept their illogical, disproved ideas and continue in mindless tradition. Also, creationists are motivated, and biased, by the fact that their theories of Biblical Inerrancy require the world to be an implausible 6000 years old.
Counterexamples
The growing list of counterexamples to the theory of an Young Earth includes: