Age of the Earth
The age of the Earth is generally agreed by scientists to be 4.54 billion years (4.54*109), plus or minus about 1%. That estimate has been made mainly through radiometric dating of Earth rocks and meteorites.   
If a piece of string 2.5 cm long (about an inch) represents one year, for example, then a 183-cm length (about 6 feet) is equivalent to the average lifetime of a person living in the United States. A string representing all of recorded human history would be fully a kilometer long, but a piece representing 4.5 billion years would be 114,280 km [71,010 miles] long!
The main geological evidence is found from lead, as there is assumed to be none of the original crust left due to erosion.
- Age of the Earth. U.S. Geological Survey.
- Dalrymple GB. (2004) Ancient Earth, Ancient Skies: the Age of Earth and its Cosmic Surroundings. Stanford University Press. ISBN 9780804749336. | Google Books preview.
- Dalrymple GB. (1991) The Age of the Earth. Stanford University Press. ISBN 9780804715690. | Google Books preview.
- Chris Stassen. Age of the Earth. The TalkOrigins Archive