Radioactivity is independent of the time the radioactive element was produced. If i remember correctly (which is a big IF, correct me if I'm wrong) this has to do with the collapse of the wavefunction into a definite state by "measurements" and then slipping back into a wave to evolve again with determinism by the Schrodinger equation. Future measurements would find the particle is decayed or not with a certain probability. Quantum Zeno effect has been observed in that repeated "measurements" are able to slow decay of various excited states. So if we have a large dense lump of some radioactive isotope how does decay rate or its lifetime not depend on the amount of stuff in it. Wouldn't macroscopic section be able to "measure" (I'm using the quotes since I'm not entirely sure what constitutes as measurements) other sections repeatedly as to make that section not decay or slow it down. Density would also come into play by making the "measurements" more frequent I guess right?