Changes

Even the staunchest advocates of carbon dating admit that it gives errors for material that the technique cannot give accurate numbers if the sample is younger than 150 years or older than 50,000 years. The errors inaccuracy for material younger than 150 years is ascribed to changes in the ratio of C-14 to C-12 in the atmosphere.<ref>http://www.talkorigins.org/indexcc/CD/CD011.html</ref>

Since the half-life of C-14 is only 5715 years, after 50,000 years only about 1/500th of the C-14 remains - and since even initially it is only present as a minute proportion of the whole (0.0000000001% of all Carbon atoms), measuring the exact quantity present accurately precision enough to be of use for dating purposes is extremely difficult. For this reason, scientists do not generally attempt to carbon date material that is believed to be older than about 50,000-60,000 years old.