Since radioactive isotopes decay at different rates how is radiocarbon dating of any value in determining how old a fossil is?

Different radioactive isotopes do decay at different rates, but radiocarbon dating is only concerned with one isotope- Carbon-14. Carbon-14 decays at a constant rate, so researchers can use it as a reliable indicator of the age of a fossil up to about 70,000 years. Older fossils require different methods to determine their age.