I thought Brother Tim was saying that the copy from around 250 years after the completion of the NT was the earliest in existence. Are you saying that there are surviving copies of the Pentateuch that are earlier?

But unless I'm seriously misreading you, your logic is very flawed in any case. Manuscripts simply did not survive long (they still don't), the materials would wear away and finally disintegrate. The only copies of any early writing that we have just happened to be preserved for one reason or another against the ravages of time. The Dead Sea Scrolls survived so well because they were preserved in jars in caves. Otherwise we have no reason to expect that anything from before the NT era would have survived.

We have ABSOLUTELY NO copies of the New Testament earlier than the Alexandrian texts on which the new versions are based -- I've heard both 2nd century and 3rd century for those -- but we certainly assume those were preceded by not only the originals but many hundreds of copies of them, and that the only copies we have of any of the Greek texts are simply some that happened to survive.

So why would you assume that there was no Septuagint in existence before the few copies that happen to have survived?

Sounds to me like Origen simply happened to be the one who wrote about the Septuagint. Certainly doesn't mean it originated with him or in his time.