Another aspect of the 6000 year old earth and the 4.5 billion year old earth,
Suppose we are determining this from the decay of U-238, we can determine the probability, based on our observations, that the actual age is 6000 years versus 4.5 billion years. of course, we need to build in error bars into each, so maybe go 5500-6500 years vs 4.3-4.7 billion years.
So, let A be our observations so far, B be the event that the Earth is 5500-6500 years old and C the probability that it is 4.3-4.7 billion years old.
We want to compare P(B|A) and P(C|A). The parameter in this is the decay rate.
And, of course, when such probabilities are computed (do you want me to do this?), the first probability is incredibly small (but not 0) and the second is well above 95%.
The basic mistake you are making is the assumption that all possibilities are equally likely: in other words, that the probability is uniform. But this is definitely NOT the case.
The decay rate is a continuous variable (so, infinitely many possibilities) and we can compute the probabilities of various observations based on this variable. And the different possibilities are NOT uniformly distributed: it is like a weighted coin of loaded dice.