Some mares produce foals late in life that as good as those she had earlier (Somethingroyal had Secretariat at 18, Sultry Sun foaled her 5th SW at 25), but many others don't. Mares, IMHO, need to building themselves back up nutritionally and physically after each pregnancy and delivery and if they don't quite make it, each subsequent pregnancy depletes her reserves even more.
With stallions, it is more likely one of a couple of causes that keeps the better proven mares away. Older stallions tend, over time, to have lowered fertility and a mare owner may not want to risk an open year, especially if he needs the income from the sale of her foals. And there is the perception problem; if he is a good sire but no Storm Cat, the mare owner may prefer to try a newer stallion who might just turn into a Storm Cat. Of course, most of the time the newby isn't a Storm Cat and is sometimes a total dud and the mare would have had a better runner with the old proven sire, but the breeder is buying and trying to sell sizzle, not steak.
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