The age of an extinct organism can be dated with the help of radioactive Carbon-14 isotope present in the sample of the organism or the rocks . Radioactive carbon isotope undergoes radioactive decay reaction at a constant rate with a half life of 5730 years. Radiometric dating is used to study the ratio of radioactive C-14 isotope present in the fossilized sample using the half life and the amount of C-14 left in the sample after the decay. So, these radioactive decay reactions (nuclear reactions) of the radioactive elements present in the sample can be employed to get an estimate of age of the organism.