Vagon submitted this awesome list over at AJ's blog (Pevensie15) and I thought it was worth reproducing here.
This is how we know the Earth is old;
Amino acid racemization - which is a technique that is used to date fossilized objects up to several millions of years in age.
Coral - whose formations take a long time to grow
Continental Drift - Tectonic drift is an incredibly slow process, the separation of landmasses would have taken millions of years. This is verified by satellite measurement.
Cosmogenic nuclide dating - The influx of cosmic rays onto the earth continually produces a stream of cosmogenic nuclides in the atmosphere that will fall to the ground.
Dendochronology - which is a method of scientific dating based on annual tree growth patterns called tree rings.
Distant starlight - Because the speed of light is finite, when you look at an object, what you are actually seeing is how the object was in the past. If the universe is only 6,000 years old how can objects billions of light years away — and therefore billions of years old — be seen?
Erosion - Many places on earth show evidence of erosion taking place over very long time periods, not drastic, as would have been caused by a worldwide flood.
Fission track dating - which is a radiometric dating technique that can be used to determine the age of uranium containing crystalline minerals.
Geomagnetic reversal - which is a change in the polarity of the earth's magnetic field. Around 171 reversals are geologically documented, which would make the earth at least several millions of years old.
Helioseismology - The composition of the sun changes as it ages.
Human Y-chromosomal ancestry - Analysis has shown that man lived around 60,000 years ago.
Ice Layering - Currently the greatest number of layers found in a single ice sheet is over 700,000, which clearly contradicts the idea of an earth less than 10,000 years old.
Impact craters - Asteroid impacts as big those that have been discovered would have led to the extinction of all medium to large size species (an event that is seen in the fossil record).
Length of the prehistoric day - as measured by evidence in coral.
Lunar retreat - which can't corelate with a 6,000 year old earth
Naica megacrystals - Based on classical crystal growth theory these crystals are older than one million years.
Oxidizable Carbon Ratio dating - is a method for determining the absolute age of charcoal samples with relative accuracy. One can determine ages of over 20,000 years ago with a standard error under 3%.
Permafrost - The formation of permafrost (frozen ground) is a slow process.
Petrified wood - The process in which wood is preserved by permineralization, commonly known as petrification, takes extensive amounts of time.
Radiometric decay - is the constant predictable decay of unstable atoms into more stable isotopes or elements. Measurements of atomic decay are generally considered to be one of the most accurate ways of measuring the age of an object, and these measurements form the basis for the scientifically accepted age of the earth. There are many different variations of the radiometric dating technique such as radiocarbon, argon-argon, iodine-xenon, lanthanum-barium, lead-lead, lutetium-hafnium, neon-neon, potassium-argon, rhenium-osmium, rubidium-strontium, samarium-neodymium, uranium-lead, uranium-lead-helium, uranium-thorium, and uranium-uranium, of which every single one will date objects far older than 10,000 years.
Relativistic jetsare jets of plasma that gets ejected from some quasars and galaxy centers that have powerful magnetic fields.
Rock Varnish - is a coating that will form on exposed surface rocks. The varnish is formed as airborn dust acumulates on rock surfaces. This process is extremely slow.
Space weathering - is an effect that is observed on most asteroids. This dating technique exceed millions of years.
Sedimentary varves - are laminated layers of sedimentary rock that are most commonly laid down in glacial lakes. The Green River formation in easter Utah is home to an estimated twenty million years worth of sedimentary layers.
Stalactites - These formations take extremely lengthy periods to form; the average growth rate is not much more than 0.1 mm per year.
Thermoluninescence dating - is a method for determining the age of objects containing crystalline minerals such as ceramics or lava.
Weathering rinds - are layers of weathered material that develop on glacial rocks. Certain weathering rinds on basalt and andesite rocks in the eastern United States appear to have taken over 300,000 years to form."