If you look at the history of Josephus, he holds to a biblical timeline, but his numbers vary in many ways that do not alter the general timeframes by more than perhaps a few centuries. What this means, then, is that either Josephus was careless with his numbers, or else the Hebrew Bible manuscripts we have today are the result of universal carelessness with the numbers. And don’t even get me started on the Samaritan Pentateuch or Septuagint–both of them had systematically altered ages. So it doesn’t seem like the age of the earth was a real big issue until Darwinism existed, when many people become much more fixated on exact dates.

The insistence on YEC for to be a ‘real Christian’ is kind of disturbing to me. Now, I personally think that anyone who isn’t YEC is probably wrongly. But anyone who preaches the gospel of Jesus Christ is on my team, or rather, His team.
There’s no doubt there’s some things many early writers were wrong about, and I definitely wouldn’t bet my soul that they correctly worked out Creation.

I don’t think YEC is new, but the insistence of it to be orthodox is. I note that many of the early writers thought that they were living in the last of the 6000 years, reading for the 2nd Coming. Further, many thought that the earth was the center of the universe.