Helium is not light enough to escape the Earth's gravity (unlike hydrogen), and it will therefore accumulate over time.
This involves measurement of three isotopes of lead (Pb-206, Pb-207, and either Pb-208 or Pb-204).
(I believe this argument was originally put forth by Mormon young-Earther Melvin Cook, in a letter to the editor which was published in .) But helium can and does escape from the atmosphere, at rates calculated to be nearly identical to rates of production.
In order to obtain a young age from their calculations, young-Earthers handwave away mechanisms by which helium can escape.
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).
he generally accepted age for the Earth and the rest of the solar system is about 4.55 billion years (plus or minus about 1%).