It is true that aged brass----as in LC Match 1967, is harder than brand new brass. But, the only difference that might exist on brass that has been resized and then left a couple weeks is that metal has a memory. When you size it, you are usually are "squeezing" it past the targeted dimension, and then it springs back a little. How much it springs back is partly dependent on the hardness of the brass to start with. What the BR guys might be talking about, and TCJ mentioned, is that over time the brass has gone back to its memory and the neck has grown very slightly, changing neck tension over long periods of time. This would be more pronounced in some brass than others, depending partly on its state of annealment or lack thereof. If you were to anneal it just before sizing, you should find very little difference in size/neck tension over time.
I know this example I am going to give here relates to steel in particular, but some of the basic principles are the same, and this example will give you an idea about metal "memory." I pulled a trailer for about 3 years, the trailer was about 12 years old at the end of that time. It was a heavy equipment trailer with a rating of 75 ton load bearing. It was built with an arch in the deck, to help carry the weight without sagging. We used it to carry loads heavy on the left side for about 5 years before this story took place. At the end of the third year I pulled it, the left side was noticeably less arched than the right side----by a good 4" in the center of a 34' arch. I requested that the trailer be re-arched and strapped to cure this ailment. It sat idle for 4 months before the welding shop got to it. When they measured it out, the difference in arch between the left and right sides was only 1/2". In the four months of resting, the left beam relaxed into the "memory" it had of the original arch, making it almost even with the right beam once more. Thus, the weld shop only had to strap the beam to keep it in its proper arch.
Metal that has been deformed by stress will return nearly to its original pre-stress position when left unstressed for a period of time, as long as the stress deformation is not past certain limits. In a sizing die, you are trying to stress the casing past the point of memory return, to create a new memory point. If the brass is soft enough, it should stay that way, or much closer to that way, than if it is a certain hardness. If it is too hard, it will crack. Thus, "aging" brass as the OP is asking, may or may not change in neck tension/size. It really comes down to the exact hardness of the brass to determine the effects of this "aging."
