The older the material, the less C-14 it will contain, as the C-14 will have decayed over time at a fairly regular rate. C-12 to C-14 ratios are somewhat variable in normal samples.
Plants take up Carbon as both isotopes, and incorporate that Carbon into their cells.
Understanding the decay of carbon-14 could also help researchers unravel other mysteries of matter.
It is limited by constraints mentioned, and by itself gives only a fuzzy approximation.
Also note, you cannot Carbon date glass, metal, or pottery. You could perhaps find a bit of leftover plant material from the crock, wine residue from the bottle, or blood on the knife.
The actual measurement of the ratio is quite accurate. If someone told you that they have Carbon dated a sample to October 13, 1492, your BS detector should immediately start flashing.
Carbon dating is an excellent tool, along with many others, to determine age and chronology.
The most stable artificial radioisotope is 11C, which has a half-life of 20.334 minutes.
All other radioisotopes have half-lives under 20 seconds, most less than 200 milliseconds.
That long half-life makes carbon-14 a useful tool to determine the ages of skeletons and other artifacts.
And while the carbon dating technique is well known and understood (the ratio of carbon-14 to other carbon isotopes is measured to determine the age of objects containing the remnants of any living thing), the reason for carbon-14's slow decay has not been understood.
Animals eat plants, and incorporate the Carbon into their cells. So the ratio should be similar throughout the food chain.
But, since C-14 is primarily created in the atmosphere by the interaction of cosmic rays with Nitrogen, the actual fluctuation of cosmic ray intensity can alter the natural abundance.
Example: I have found a bit of bone in a strata with pottery shards from a known culture, and a bit of charcoal from the same strata.