I am not a stock maker, but have had a love affair with walnut for 50 years. I know that antique restorers and museum support artisans greatly prize old wood/veneer that they can get their hands on. Borers, powder post beetles are more a problem in old wood/furniture than moisture- especially in a climate controlled environment.
Once the "bound moisture" in the wood cells has "left", the wood will largely acclimate to its host environment; it will not continue to lose moisture beyond the relative humidity of its environment.
I have in the safe a French blank that's appx. 40 years old, with an MC (moisture content) perfect for working (9-12 % is a good range). So I don't believe there is a "use by" issue, though you should be careful if the blank has been in a desert environment and your stocker is in Florida.
At the end of the day, get your stockmaker's opinion of the blank and its suitability for the project.
Mike
