Using relative and radiometric dating methods, geologists are able to answer the question: how old is this Third, magnetism in rocks can be used to estimate the age of a fossil site. . However, if the bone is 70, years or older the amount of 14C left in the bone will be too small to measure accurately. .. Earth Systems. Paleontologists still commonly use biostratigraphy to date fossils, often carbon- 14 in bones or a piece of wood provides an accurate date, but. Carbon dating is a technique used to determine the approximate age of once- living Before the twentieth century, determining the age of ancient fossils or called radioactive dating, to obtain a very accurate measurement of the age of Earth.