Best rock for radiometric dating
For instance, carbon-14 has a half-life of 5,730 years.
After an organism has been dead for 60,000 years, so little carbon-14 is left that accurate dating cannot be established.
These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace.
The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created.