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 Earth is very old. But how old, exactly? And how can we know with any degree of confidence? As Henry Reich describes in the video above, the process of scientifically estimating the age of the Earth revolves around, essentially, finding the oldest piece of the planet we can, then figuring out how old that piece is.