How to estimate the age of earth

They don't estimate that it formed 4500 billion years ago--the universe is not that old. They estimate that the earth formed 4.5 billion years ago. This is based on the radioactive decay of elements within rocks--in this case rocks found in Australia.

Click here to read a Wikipedia article on this subject.

Yes I was mistaken,the earth was formed about 4500 million years ago.Thanks for your wiki link.

we use lead and uranium isotopes from stable zircons in old formations but often those values can be dubious random numbers without some calibration techniques. We therefore use meteorites as "a fence' around which we can determine just what is the oldest age we can find in the solar system. Presumably the Solar System is a "partner" in the age pf the earth. Zircon radioactive clocks can be "Reset' by reheating or by loss of lead in the crystal, so we may never find anything older than some of those crystals from the Northern Canadian Shield or Australia. Look up the HOLZINGER OCTAHEDRITE or DIABLO CANYON METEORITE and the age of the earth,,

PS, Kumar, I like your signature, it is straightforward