The most direct means for calculating the Earth's age is a Pb/Pb isochron age, derived from samples of the Earth and meteorites.This involves measurement of three isotopes of lead (Pb-206, Pb-207, and either Pb-208 or Pb-204).
The higher the uranium-to-lead ratio of a rock, the more the Pb-206/Pb-204 and Pb-207/Pb-204 values will change with time.
If the source of the solar system was also uniformly distributed with respect to uranium isotope ratios, then the data points will always fall on a single line.
The actual underlying assumption is that, if those requirements have not been met, there is no reason for the data points to fall on a line.
The resulting plot has data points for each of five meteorites that contain varying levels of uranium, a single data point for all meteorites that do not, and one (solid circle) data point for modern terrestrial sediments.
Helium is not light enough to escape the Earth's gravity (unlike hydrogen), and it will therefore accumulate over time.
The current level of helium in the atmosphere would accumulate in less than two hundred thousand years, therefore the Earth is young.While these values do not compute an age for the Earth, they do establish a lower limit (the Earth must be at least as old as any formation on it).This lower limit is at least concordant with the independently derived figure of 4.55 billion years for the Earth's actual age.Further, the processes of erosion and crustal recycling have apparently destroyed all of the earliest surface.The oldest rocks which have been found so far (on the Earth) date to about 3.8 to 3.9 billion years ago (by several radiometric dating methods).Note that young-Earthers cannot accuse us of selective use of data -- the above table includes a significant fraction of all meteorites on which isotope dating has been attempted. 286) , less than 100 meteorites have been subjected to isotope dating, and of those about 70 yield ages with low analytical error.