Age of the Earth.

http://en.wikipedia.org/wiki/Age_of_the_Earth

The age of the Earth is 4.54 ± 0.05 billion years (4.54 × 10⁹ years ± 1%).This age is based on evidence from radiometric age dating of meteorite material and is consistent with the ages of the oldest-known terrestrial and lunarsamples. Following the scientific revolution and the development of radiometric age dating, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old.

The oldest such minerals analyzed to date – small crystals of zircon from the Jack Hills of Western Australia – are at least 4.404 billion years old.Comparing the mass and luminosity of the Sun to those of other stars, it appears that the solar system cannot be much older than those rocks. Calcium-aluminium-rich inclusions  – the oldest known solid constituents within meteorites that are formed within the solar system – are 4.567 billion years old,giving an age for the solar system and an upper limit for the age of Earth.

It is hypothesised that the accretion of Earth began soon after the formation of the calcium-aluminium-rich inclusions and the meteorites. Because the exact amount of time this accretion process took is not yet known, and the predictions from different accretion models range from a few millions up to about 100 million years, the exact age of Earth is difficult to determine. It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages.