How old is the Earth?

[theVOID]

Well if the Earth is really 4.5 billion years old then you cannot "observe" that decay rates are constant because yoru observation is vastly too small and insignificant compared to the whole time period. Even if you could observe it for 100 years it would still only be 2.2X10^-11 percent of the total time. Even a curved line looks straight when you only observe an insignificant portion of it. So you're going to have to provide some other backing as to how you know those rates are constant.

Sit down and learn newbie.

Saying that a particle decays on average after 10^8 years also means that if you have a group 10^8 particles you will see one decay every year because even though the average particle decays ever 10^8 years, there is a probability that 1 in a set of 10^8 particles will decay in within 1 year, if you had 10^80 particles you would get 10 decays a year, and 10^800 100 decays, and this idea applies to all decay rates. That's not a hell of a lot of particles comparatively, you could quite easily measure decay rates to a really high degree of accuracy by seeing rate of decay in large sets of particles.