Those who don’t accept the Biblical account of history look for other ways to discover the age of things.
U238 is the "parent" isotope, and Pb206 is the "daughter" isotope.
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.
Scientists found that by measuring the amounts of both parent and daughter isotopes in matter (seeing how much of the U238 has stabilized into Pb206), they could accurately calculate the age of that matter.
For example, it takes 4,460,000,000 years for half a sample of U238 to turn into Pb206.