Scientists first make observations that raise a particular question. In order to explain the observed phenomenon, they develop a number of possible explanations, or hypotheses. This is the inductive part of science, observing and constructing plausible arguments for why an event occurred.
As indicated earlier, the Bible does not fix the age of the earth, contrary to the claims of Answers in Genesis. Historically, their claim comes from the work of James Ussher, Bishop in the Church of Ireland, from 1625 to 1656.
Hadean Eon [4500 Myr - 4000 Myr ] The age of the Earth is projected to be about 4500 Myr from radiometric dating of the oldest rocks and meteorites. There is evidence of a time of intense bombardment of the Earth in the time period from about 4100 to 3800 Myr in what is called the late heavy bombardment.