I still don't see how it is a constant? How do i determine what is constant and what is not?
Physicists understand radioactive decay very well. They can predict what we would see today if radioactive decay rates varied in the past. Since the physics of atomic nuclei is a very fundamental part of how our universe works, it turns out that there are lots of things we would see today if radioactive decay rates varied in the past. We've looked really hard for those things, and they aren't there. We conclude (we do not assume) that radioactive decay rates have been constant for 13-ish billion years.
What does the constant decay rate has to do with the age of the earth? What if the decay rate change, then what happen?
If decay rates changed, it would cause the radiometric dating methods that tell us about great ages to be wrong. If radioactive decay rates sped up the ages we would get would be older than the real age. If the Earth is really around 10,000 years old that would require speeding up radioactive decay rates so much that the Earth would melt and everyone would be fricaseed by the heat. And the radiation would fry 'em too.
Another fact is that there are many different ways that radioactive elements decay. If our methods give us the wrong answers than all those differnt ways would have to change in just the right way in a coordianted manner. Otherwise dating methods based on different modes of decay wouldn't agree so often.