Radiocarbon dating age of the earth
For example, with the uranium/lead method scientists have attempted to estimate what the original ratio (of uranium-238 to lead-206) was when the Earth formed.To do this they have selected a certain meteorite, which contained various types of lead (including lead 204, 206, 207 and 208) but no uranium, and they have assumed that this ratio is equivalent to the earth's original lead ratio.
And since the earth is not a closed system, these last two assumptions make radiometric dating highly subjective and questionable.
For example, if a rock sample was below the water table at any time, leaching would take place.
For Uranium/Lead dating this means that some of the uranium that was initially present would be "leached" out of the rock.
Radiocarbon decays slowly in a living organism, and the amount lost is continually replenished as long as the organism takes in air or food.
Once the organism dies, however, it ceases to absorb carbon-14, so that the amount of the radiocarbon in its tissues steadily decreases.
For a more detailed discussion of this and why a Creator must have been intimately involved with the Creation of Life see: A Closer Look at the Age of the Earth.