From
http://www.talkorigins.org/indexcc/CF/CF210.html:
The constancy of radioactive decay is not an assumption, it is supported by evidence:
1. The decay rates of radioactive nuclides used in radiometric dating have not been observed to vary since their rates were directly measured. There have been experiments that attempted to change decay rates that didn't work. Extreme pressure can cause electron-capture decay rates to increase slightly (less than 0.2 percent), but the change is small enough that it has no detectable effect on dates.
2. Supernovae produce large quantities of radioactive isotopes. These isotopes produce gamma rays with frequencies and fading rates that are predictable according to present decay rates. These predictions hold for supernova SN1987A, which is 169,000 light-years away. Therefore, radioactive decay rates were not significantly different 169,000 years ago. Present decay rates are likewise consistent with observations of the gamma rays and fading rates of supernova SN1991T, which is sixty million light-years away, and with fading rate observations of supernovae billions of light-years away.
3. The Oklo reactor was the site of a naturally occuring, spontaneous nuclear reaction 1.8 billion years ago. The fine structure constant affects neutron capture rates, which can be measured from the reactor's products. These measurements show no detectable change in the fine structure constant and neutron capture for almost two billion years.
4. Radioactive decay at a rate fast enough to permit a young Earth would have produced enough heat to melt the Earth.
5. Different radiometric dating techniques give consistent dates for the same rock. Different radioisotopes decay in different ways and at different rates. It is highly unlikely that a variable rate would affect all the different mechanisms in the same way and to the same extent. Furthermore, radiometric dating techniques are consistent with other dating techniques, such as dendrochronology, ice core dating, and historical records.
6. The half-lives of radioisotopes can be predicted from first principles through quantum mechanics. Any variation would have to come from changes to fundamental constants. According to the calculations that accurately predict half-lives, any change in fundamental constants would affect decay rates of different elements disproportionally, even when the elements decay by the same mechanism.
We know the Earth is about 4.55 billion years old because...
read this:
http://www.talkorigins.org/faqs/faq-age-of-earth.html
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