Which Bible verse states that Adam's age was calculated from the time he was cast out of Eden?
There is none and neither is there a verse to the contrary.
The point made is that those that attack ‘Young Earthers’ are engaging in nonsense just as Young Earthers are. The Bible says nothing about a 6000 year old Earth or provides clear reference that establishes a 6000 year old Earth. The Bible is still consistent.
That said, there are references in the Bible that scholars propound from saying that Adam lived ‘physically’ for 900+ years after expulsion from the Garden.
There are clear references that Adam was formed in ‘spirit’ before the creation of the Garden of Eden. In other words Adam came into existence before the Garden existed.
So there are ‘gaps’ that leave mysteries that cannot be solved. In my view any person or organization that endeavors to prove the Earth is young or prove the Bible is false in revealing the Earth is young (the Bible does not reveal this), that these endeavors are an exercise in futility.
My view is that those engaged in proving the Earth is young are misguided and those that attempt to prove the Bible is false are opportunists who seek to distort facts or refer to facts which are not actual facts.