​​​​​​Nonetheless, I still don't understand why the hide of an older cow should be preferred to that of a young one, once they reach full size. They're not trees-- they don't have annual growth rings. As far as I know, the only thing that happens to an animal's hide, once it gets to maturity, is damage.

So I have a hard time believing that it's the cows' fault the leather isn't any good. The animals haven't changed. If there's a real difference in the quality of the leather (which I don't doubt) the problem is in the tanning.