Since 1955 the estimate for the age of the Earth has been based on the assumption that certain meteorite lead isotope ratios are equivalent to the primordial lead isotope ratios on Earth.

In 1972 this assumption was shown to be highly questionable.

Uranium comes in two common isotopes with atomic weights of 235 and 238 (we'll call them 235U and 238U).

What makes this fact useful is that they occur at different rates, as expressed in their half-lives (the time it takes for half the atoms to decay).However, rocks and other objects in nature do not give off such obvious clues about how long they have been around.So, we rely on radiometric dating to calculate their ages.So, if you know the radioactive isotope found in a substance and the isotope's half-life, you can calculate the age of the substance. Well, a simple explanation is that it is the time required for a quantity to fall to half of its starting value.So, you might say that the 'full-life' of a radioactive isotope ends when it has given off all of its radiation and reaches a point of being non-radioactive.