Lead 204 is entirely primordial and the other isotopes found on earth would be found at roughly the same concentration were all of the lead on earth primordial. It’s the excess ratios of the other isotopes of lead that can be attributed to radioactive decay. That is a substantial proportion of the lead on earth, but to say the “existence of lead” is proof of the age of the earth is entirely incorrect.

en.wikipedia.org/wiki/Isotopes_of_lead