Different forms of this element are often used in a type of radiometric dating that determines the ages of objects up to 40,000 years.

It is the carbon element whose different forms are often used in radiometric dating to determine the ages of objects. In the radioactive decay, radioactive carbon breaks down to form stable isotope.  It is specifically the carbon-14 (C-14) isotope which combines with O2 to form of carbon dioxide (CO2).  

If we know the half-life of C-14, the decrease in mass of C-14 in the organic matter can be related to the time period taken to reach that particular mass and in this way age of organic matter can be determined