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.
The age of Earth was calculated to be about 24-40 million years based on the laws of thermodynamics. Unknown at the time was that the Earth has an internal heat source (radioactive decay) In 1899, John Joly (Irish) calculated the rate of delivery of salt to the ocean.