For a long time, the Earth was thought to be only a few thousand years old. This was in line with Biblical teaching. The first "jump" in the estimates of the age of the earth were made by James...
1. Most estimates based on a literal interpretation of the Bible, and in particular the belief that the Hebrew word : "yom" in Genesis refers to a 24 hour day, cluster around 6,000 years :' 2,000...
Here is a website that purports to calculate a biological age. They don't explain their methodology, but it seems to be calculated as something like: Your current age MINUS your personal life...