If by relativity you mean the special theory, it does not even formally apply to the universe at large; only general relativity does. There is no particularly good reason why a layman should consider special relativity more suitable for describing cosmology than general relativity. There are plenty of observational reasons why general relativity is the best way to do so. Only general relativity gives you Big Bang cosmology and all of the predictions that flow from this theory. Special relativity gives you wrong answers.
Can there be regions of the universe with different ages? Only if the Big Bang was a very anisotropic 'explosion' with many delayed mini-bangs. This would be seen in a pronounced anisotropy in the cosmic background radiation which simply is not seen.
Therefore, the regions in the visible universe we now study ought to all be of the same age measured since the Big Bang. Logic and observation seem to demand nothing less than this at the present time.
Copyright 1997 Dr. Sten Odenwald
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