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.
To summarize: A generation in the bible is primarily the age of a man when his first male child is born; but secondarily, the age of a man at his death. The bible then rounds the exact span of years in a generation to be 40 and 100 years as the primary usage, but on occasion, 70 years. And 70 is the average of 40 and 100, which allow the three timeframes to work in numeric harmony to produce the numeric symmetry found throughout the bible.