The age of the Earth and the universe has nothing to do with evolution, except that creationists need to disallow both an old Earth and evolution to retain their faith.
While I entirely agree with you that the scientific theory of Evolution has nothing to do with the age of the earth, or the universe, or even the origin of the species or the history of how we came to be.
However, as an evolutionist pointed out to me a few weeks ago on these boards, evolution can’t work in short periods of time, which is why we can’t observe it creating new species, but if you just give it millions of years then it is a fact.
Evolution is a scientific impossibility if the earth is too young.
On the other hand, a young earth is not a necessity for the creation story, it just adapts easier to history as described in the Bible.
I don’t believe the earth or the universe is old, but I also don’t believe the earth is 6000 years old. Actually, I have no firm belief on how old the earth is, because looking at all the evidence, I don’t see a reason to accept as fact any of the numbers.
It takes a year to build a skyscraper. If someone created one out of thing air, but we lost all knowledge of how it was created, it would look like the top floor was put on a year after the bottom floor, because that is what science would say is necessary.