Because of this relatively short half-life, radiocarbon is useful for dating items of a relatively recent vintage, as far back as roughly 50,000 years before the present epoch.Radiocarbon dating cannot be used for older specimens, because so little carbon-14 remains in samples that it cannot be reliably measured.
Uranium 235 decay to lead has a half-life of 713 million years, so it is well suited to dating the universe.
other isotope pairs cover intermediate time periods between the spans for carbon 14 and uranium.
Some radiometric dating methods depend upon knowing the initial amount of the isotope subject to decay.
The ratio of normal carbon (carbon-12) to carbon-14 in the air and in all living things at any given time is nearly constant.
Maybe one in a trillion carbon atoms are carbon-14.