Roughly how old is a skeleton that has lost 90% of its carbon-14?
We define the rate of this radioactive decay in half-lives.
If a radioactive isotope is said to have a half-life of 5,000 years that means after 5,000 years exactly half of it will have decayed from the parent isotope into the daughter isotopes.
Potassium-40 on the other hand has a half like of 1.25 billion years and is common in rocks and minerals.
This makes it ideal for dating much older rocks and fossils.
These people have only succeeded in correctly showing that one can fool a single radiometric dating method when one uses it improperly.