[metmom]

Some specific isotopes are created specifically from this decay. They aren't isotopes found in nature outside this process.

The presumption, then, is that there were none of those isotopes at one point. How is that point in time determined?

Wouldn't the material have started decaying as it was formed? Unless it all formed at once, there would be no accurate way to determine the age, as some of the parent material would already have been decaying as other of the material would be forming.

[LeGrande]

Wouldn't the material have started decaying as it was formed? Unless it all formed at once, there would be no accurate way to determine the age, as some of the parent material would already have been decaying as other of the material would be forming.

Yes, but your reasoning is backwards.

The radioactive material (all heavy matter in fact) came from a supernova at which point it started decaying.

The question you should be asking is what does that date, except for the supernova?