One of the ways that researchers measure the age of organic material is through carbon-14 dating.
C14 is a radioactive isotope that's made when cosmic rays bombard nitrogen atoms at high altitudes, converting them to this excited form.
Then creatures that can't make their own food through photosynthesis (like us) eat the ones that can, and that C14 is taken into our bodies as well.
We know that it takes 5,730 years for half of the C14 in a sample to decay. And if we compare the amount of C14 in a dead thing to the amount of regular carbon-12, voila! Now, some people who think that the earth is only 6,000 years old may base their claims on words in the Bible, not measurable evidence. If I wanted to find out the age of a dinosaur fossil, I might measure its uranium-235 concentration, which has a half-life of 704 million years.It takes another 5,730 years for half of what's left to decay, and so on. And one ploy they use to cast doubt on radiocarbon dating is to point out its shortcomings. So, anything older than 50,000 years only has too little C14 left to make an accurate calculation of its age. Radioactive isotopes like potassium-40 and rubidium-87 have half-lives in the billions of years.When some living things, like plants and algae, make their own food through photosynthesis, they take in carbon dioxide from the air.Trace amounts of C14 make up a tiny percentage of that carbon dioxide, and it's integrated into the tissues of the organism.
Inscriptions, distinctive markings, and historical documents can all offer clues to an artifact's age.
And if the artifact is organic—like wood or bone—researchers can turn to a method called radiocarbon dating.