All radioactive isotopes with half-lives greater than 75 million years are found on Earth, while not a single radioactive isotope with a smaller half-life is found in nature (except for a few which are found as decay products). This supports the claim that the Earth is old enough for the shorter-lived isotopes to have decayed away (which takes many times the half-life), but it would be a strange coincidence indeed if the young-Earth hypothesis were correct. Furthermore, we have very clear evidence that several of these short-lived radioisotopes did in fact exist early in the Solar System's history (aluminum-26 being the most abundant and well-known). Since Al-26 is known to have existed in the past (due to detected excesses of its decay product, Mg-26, in ancient meteorites), and has completely decayed away since the beginning of the Solar System, the age of the Solar System is demonstrated to be at least many times the half-life of Al-26 (which is 26,000 years). Among the radioisotopes for which the same argument applies are hafnium-182 and palladium-107, with half-lives greater than 10 million years.