I understand how carbon dating works, though I do not understand why it doesn't happen while a creature is living. Because while we are alive we still have carbon 14 in us, so shouldn't it work?

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    $\begingroup$ It should and does. It will correctly show your age as 0. $\endgroup$ – Ivan Neretin Nov 12 '19 at 6:16
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    $\begingroup$ Even though this is a good question, I'm tempted to close this as a duplicate of How does radiocarbon dating work? Knowing the principles of the analysis would ultimately answer this question too, and that answer indeed does. $\endgroup$ – andselisk Nov 12 '19 at 9:34
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    $\begingroup$ An error rate of about +/- 100 years is not all that useful in determining the age of a living creature. $\endgroup$ – vsz Nov 12 '19 at 17:31
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    $\begingroup$ @vsz, not everything that lives is a creature. Some plants live for thousands of years, but... Radio carbon dating will not tell you the age a living tree. Not even if you could read it with absolute precision. The radio carbon clock does not start ticking until the organism dies. (see Poutnik's answer, below.) $\endgroup$ – Solomon Slow Nov 12 '19 at 20:36
  • $\begingroup$ A general comment to integrate the answers below. The ratio between C isotopes is constant within the atmosphere. Decaying C14 is constantly replaced. $\endgroup$ – Alchimista Nov 13 '19 at 9:09

There are plenty of good sources online explaining the principle behind radiocarbon dating. For instance, the wikipedia explains:

During its life, a plant or animal is in equilibrium with its surroundings by exchanging carbon either with the atmosphere, or through its diet. It will therefore have the same proportion of $\ce{^14C}$ as the atmosphere, or in the case of marine animals or plants, with the ocean. Once it dies, it ceases to acquire $\ce{^14C}$, but the $\ce{^14C}$ within its biological material at that time will continue to decay, and so the ratio of $\ce{^14C}$ to $\ce{^12C}$ in its remains will gradually decrease. Because $\ce{^14C}$ decays at a known rate, the proportion of radiocarbon can be used to determine how long it has been since a given sample stopped exchanging carbon – the older the sample, the less $\ce{^14C}$ will be left. [16]

Your question is somewhat subtle: why doesn't decay begin immediately once the isotopes have been incorporated into an organism? Can't measurements of part of an organism tell us when that particular organ or body part was formed?

The key limitation is explained in a document from a company (Beta Analytic) that provides biobased content / renewable carbon measurements:

Although carbon-14 is radioactively decaying away in the body, it is constantly being replaced by new photosynthesis or the ingestion of food, leaving the amount relatively constant.

But if a body part no longer cycles carbon (is in effect dead), then radiocarbon dating should be useful. One application, for instance, is in dendrochronology, the analysis of tree growth rings to assess age and location:

Dendrochronology is useful for determining the precise age of samples, especially those that are too recent for radiocarbon dating, which always produces a range rather than an exact date, to be very accurate. However, for a precise date of the death of the tree a full sample to the edge is needed, which most trimmed timber will not provide. It also gives data on the timing of events and rates of change in the environment (most prominently climate) and also in wood found in archaeology or works of art and architecture, such as old panel paintings. It is also used as a check in radiocarbon dating to calibrate radiocarbon ages.1

Combining tree samples of known age (thanks to dendrochronology) and other sources with radiocarbon ratios provides a calibration method.

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The metabolism of living creatures keeps the dynamic equilibrium of their $\ce{^{14}C/^{12}C}$ ratio with the enviromental $\ce{^{14}C/^{12}C}$ ratio via photosynthesis, breath, food and excrements.

It is not just about one time building, but also about continuous recycling of the body content.

Such an equilibrium means continuous resetting of the $t=0$ on the radiocarbon dating timescale.

After death, this dynamic equilibrium freezes and the $\ce{^{14}C}$ slowly decays according the known exponential curve.

Due minor possible fluctuations of the initial ratio, the method is not usable for short time periods without the initial ratio context.

Additionally, measurent errors affect the short time periods the most.

See more at

Wikipedia - Radiocarbon dating

Wikipedia - Radiocarbon dating considerations

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While we are alive, we are constantly replacing our molecules. With every breath cycle, you get rid of some of your carbon (CO2). With every meal, you take in new carbon (carbohydrates, proteins, lipids). The carbon in your body gets cycled as your cells die and are replaced. This replacement process stops when you die.

After this, whatever carbon remains will decay and can be used to estimate age... or really, time since the organism stopped exchanging carbon.

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  • $\begingroup$ this was really helpful! $\endgroup$ – π person Nov 13 '19 at 2:37

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