[Graham Boak]

In reply to Revi16, I do not know what precise meaning the USAAF attached to "War Weary". It may be just as you say, but that does not mean that airframes, particularly in this case fighters, do not become weary, in the more recognised sense of tired and worn. They did and do.
If you wish to argue that US aircraft would show this less because they were built to higher structural limits than those of other nations, this is probably true. However, it is not true that the problem did not exist.

A few hundred hours, for WW2 fighters pulling multiple g on a fairly regular basis, is a significant part of their life.

Drag increases from distorted panels, increased steps and gaps, and even rough paint, ARE noticeable. Just polishing an aircraft makes it go faster. Each increment may be small but they all add up. I spent some years in a design office working on jet fighters, and this is one of the messages most difficult to get across when discussing build standards. "But it is only a tiny amount..." like "It is only a small weight increase." "Just a little bit pregnant." (No, I didn't hear that one.) But there is another request tomorrow, and tomorrow, and tomorrow... We are talking about steps and gaps and distortion of the order of tens of thousands of an inch. They not only add individually small drags but affect the smooth flow over larger areas. This sort of damage occurs at a lower level than major distortions that would indeed result in scrap or at least rework. It results in higher landing speeds and hence more stress applied to the airframe in that way. They result in more drag in the cruise and hence lower range.

Reading of WW2 literature will show many examples of older aircraft being passed down from senior pilots to juniors, because older airframes being considered slower and less agile. Not just because a later model became available. In their biographies, pilots complain about being posted to training units, not least because the aircraft are old and tired. There are extreme cases such as Malta. The Russians complained (in particular) about those of the Hurricanes and Spitfires sent to them that were reworked examples that had already served tours of duty on the Western Front.

In the later years of WW2, the US did benefit from a seemingly endless supply of new aircraft that meant they could withdraw used machines that still had considerable life remaining. This no doubt reduced the exposure of US pilots to any genuinely worn machines. That doesn't mean they couldn't and didn't exist.