G., The maximum theoretical age obtainable by radiocarbon dating depends on the instrument used to do the analyses.
Theoretically, the AMS instrument should obtain ages up to 95,000 years, but practically, 60,000 years or less is the limit.
This discovery meant that there are three naturally occurring isotopes of carbon: Whereas carbon-12 and carbon-13 are stable isotopes, carbon-14 is unstable or radioactive.
Carbon-14 is produced in the upper atmosphere when cosmic rays bombard nitrogen atoms.
But the radiocarbon detected in diamonds is equivalent to ages of up to 80,000 years.
So you can see why the various age limits have appeared in different publications of ours.
Shell may succumb to isotopic exchange if it interacts with carbon from percolating ground acids or recrystallization when shell aragonite transforms to calcite and involves the exchange of modern calcite.
The surrounding environment can also influence radiocarbon ages.
Desmond Clark (1979:7) observed that without radiocarbon dating "we would still be foundering in a sea of imprecisions sometime bred of inspired guesswork but more often of imaginative speculation." And as Colin Renfrew (1973) aptly noted over 30 years ago, the "Radiocarbon Revolution" transformed how archaeologists could interpret the past and track cultural changes through a period in human history where we see among other things the massive migration of peoples settling virtually every major region of the world, the transition from hunting and gathering to more intensive forms of food production, and the rise of city-states.
However, as with any dating technique there are limits to the kinds of things that can be satisfactorily dated, levels of precision and accuracy, age range constraints, and different levels of susceptibility to contamination.
Since carbon is fundamental to life, occurring along with hydrogen in all organic compounds, the detection of such an isotope might form the basis for a method to establish the age of ancient materials.