There is no serious question about the age of the fossils
Based on what? decay rates?
Decay rates, stratigraphy, fossil correlations, ...
It's all one big, connected, consistent web of evidence. Including lots of evidence that decay rates haven't changed.
Message 53:
Observations used to establish the constancy of decay rates, include (but are not limited to):
- Observations of nuclear reactions in distant stars and distant galaxies (for which the reactions took place thousands or millions of years ago).
- Inferences about nuclear processes in the very early universe before galaxy formation.
- Cross checking of dates against other non-radiometric dating methods.
- Cross checking of radically different radiometric methods.
- Study of residues from the Oklo natural nuclear reactor, active nearly two billion years ago.
- Theory of quantum mechanics, which is itself one of the most precisely studied and tested models in physics. Radioactive decay is a process that is well understood. We know a great deal about the relevant forces and the structure of atoms, and how and why they decay. In fact, I would say radioactive decay is substantially better understood than gravity. This illustrates the principal that confidence in scientific models is related also to how well the underlying principals are understood.
- Testing of a range of conditions in which decay might vary. If decay rates have varied, then can we reproduce the conditions under which this occurs? In some cases, yes; and none of them make any difference to dating techniques.