I don't know what casting dates you have been looking at, but the VAST majority of casting date to assembly date is generally about 3 months or less apart. There of course are exceptions but usually no more than 6 months at the out side. Hemis are another story but that is simply because the sales were slow. In fact, they did not even cast any Hemi engines in 1967 because they had so many left from 1966. It is common to find 1968 model cars with 1966 casting date blocks until about mid January because they did not cast the second run of Hemis until Jan 1, 1968. Obviously the factory did not season many blocks for much more than about 3 months.
This whole thread began about choices of blocks for rebuilding and picking blocks that may have had a harder life than others and it somehow got off on this seasoned block thing. ANY block that has been installed in a car for any length of time has run all the heat cycles that any engine needs to be stable.
You are correct about the seasoning and letting the block sit for a while to stablize when new.
The two subjects are completely different things and using a used block regardless of how it was treated in the past has no advantage or disadvantage once it is machined for rebuild. The things you want to do as stated by many previously in this thread is to mag and sonic test for core shift/wall thickness/cracks if boring is necessary, which it nearly always is.