Wikipedia carbon dating
However, once the organism dies, the amount of carbon-14 steadily decreases.
By measuring the amount of carbon-14 left in the organism, it's possible to work out how old it is.
Because the cosmic ray bombardment is fairly constant, there’s a near-constant level of carbon-14 to carbon-12 ratio in Earth’s atmosphere.
Organisms at the base of the food chain that photosynthesize – for example, plants and algae – use the carbon in Earth’s atmosphere.
Carbon dioxide in the atmosphere contains a constant amount of carbon-14, and as long as an organism is living, the amount of carbon-14 inside it is the same as the atmosphere.It’s not absolutely constant due to several variables that affect the levels of cosmic rays reaching the atmosphere, such as the fluctuating strength of the Earth’s magnetic field, solar cycles that influence the amount of cosmic rays entering the solar system, climatic changes and human activities.Among the significant events that caused a temporary but significant spike in the atmospheric carbon-14 to carbon-12 ratio were above-ground nuclear test detonations in the two decades following World War II.The half-life is always the same regardless of how many nuclei you have left, and this very useful property lies at the heart of radiocarbon dating. The graph below shows the decay curve (you may recognize it as an exponential decay) and it shows the amount, or percent, of carbon-14 remaining.You will notice that after around 40,000 years (or 8 half-lives), the amount left is starting to become very small, less than 1%.