Lenses: Old Versus New - Any Significance?

Is there really any difference in the same model of lens made 12 or 15 years ago versus one that was made more recently?

Hypothetical example: A person wants to buy a 300mm EF IS f/4 and sees an add for a pristine copy with a date code of UM, which is 1998, and then sees another add for a pristine copy with a date code of UU, which is 2006. The conditions of the lenses are visually equal, so there's no reason to believe the older lens has seen more or rougher use. Let's say, for argument's sake, the older lens is $50 less than the newer lens, a nominal amount. Is there any reason why the newer lens should be preferred to the older lens, all else being equal? Are there any differences in QC or updates to electronics between old and new in any of these lenses (not counting those lenses with major updates, such as the latest 70-200 f/2.8 IS)?

As no one has replied to you query as yet, I will take a shot at it. I have also asked myself the same question. Is the cost saving of 5-10% worth buying an older lens? In practice, I have bought the newer lens as I did not consider the small savings to be worthwhile.
My first thought is that there would be some kind or wear to moving components or breakdown of electronic components over the 8 years in the older lens. As a result, the older lens is closer to a breakdown (assuming the same amount of use per year).
Secondly, one would have hope that improved algorithms and/or circuitry are being used in the newer lens for faster/more accurate focussing (although the difference might for all intents and purposes be neglible).
On the other hand, the manufacturer might be making the newer lens a bit cheaper. Less rugged components, maybe even using a lower grade of glass as they see how the original has performed during long term use.
I know that I have not answered your question but hopefully someone else can comment.

If the difference is $50, go for the newer one.

If the difference is $500, go for the older one.