The information is correct. The reason could be that the average age of the slaughtered cattle, beef cattle apx. 18 to 24 month, cows at least 3years, in some cases older than ten years.

The problem is older ...
(Rebours book from 1975, p. 113):
"Jadis, une seule selle était pratiquement utilisée par tous, c'était la BROOKS « B.17 » anglaise. En cuir de première qualité, sa forme était rationnelle et son confort, une fois bien « rodée », était parfait. Malheureusement, la qualité du cuir d'après-guerre n'est plus la même, le principe même de fabrication de cette selle, ainsi que sa forme, ont été modifiés et actuellement, il n'y a pas un coureur sur cent qui l'utilise encore.
Disons aussi que la selle classique en cuir présente un inconvénient sérieux.
Aujourd'hui, les coureurs courent presque toute l'année, et ils n'ont plus comme jadis cette longue période de creux, entre septembre et avril, où ils pouvaient « faire » une nouvelle selle, celle-ci, neuve, étant très dure et risquant de blesser le cycliste."
Translation:
"In the past, only one saddle was used by almost everyone, it was the English BROOKS "B.17". Made of top quality leather, its shape was rational and its comfort, once well "run in", was perfect. Unfortunately, the quality of the post-war leather is no longer the same, the very principle of manufacturing this saddle, as well as its shape, have been modified and at present, not one rider in a hundred still uses it.
Let's also say that the classic leather saddle has a serious disadvantage.
Nowadays, riders race almost all year round, and they no longer have that long period of time between September and April when they could "make" a new saddle, as this new saddle is very hard and can injure the rider."

but Brooks is still Brooks....

​​​​​​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.

Just spit-balling here---

Could antibiotics, hormones and other dietary conditions impact hide quality? Do the most cost effective hides come from feed-lot cattle? Would a free-range hide be different? Of course I have no way to answer any of these questions!

Just because bones aren't getting longer doesn't mean the composition of the specimen (human, cow, dog, etc) isn't changing. I'm 66 and I can tell you for sure that my skin ain't what it was when I was 26. It seems quite possible that the properties of leather made from 18-month old and ten-year old cattle might be different.

Idealé 90 never left (in my book).

Modern Brooks saddles are fine but the quality looks to be nowhere near these new Ideale saddles. Like it or not, Brooks is a relatively large company and you can see the effect that cost-savings have had on the materials and consistency of their saddles.

The real excitement for me of the Ideale model is the return of the open-nose saddle. To me looks much more natural on a vintage bike.