The principle basis for Old Earth is [[radiometric dating]]. But it is a logical tautology to assume that [[radioactive decay]] rates have always been constant, even at higher energy levels. Such assumption is identical to assuming that the Earth is old, and hence that argument is circular.
Other purported “evidence” of an old earth include: Naica megacrystals, rock varnish, amino acid racemization, dendrochronology , distant starlight, erosion , fission track dating , geomagnetic reversals , helioseismology , human Y-chromosomal ancestry, iron-manganese nodule growth, impact craters , lack of DNA in fossils, lunar retreat, oxidizable carbon ratio dating, permafrost, petrified wood, relativistic jets, , seabed plankton layering, sedimentary varves, space weathering , stalactite growth, thermoluminescence dating, weathering rinds, Baptistina asteroid family, coral growth, cosmogenic nuclide dating, length of the prehistoric day, continental drift, ice layering. Despite persistent claims by scientists, none of these methods has proved conclusive.
The discipline of [[uniformitarianism]] is also a basis for assuming an old Earth. Extrapolating the rate of geologic processes backwards in time yields an Earth substantially older than the Biblical 6,000 years. Uniformitarianism is also based on assumptions, however; namely, that all geologic processes were acting at the same or similar rates for the whole history of the Earth.