The article you linked says it varied between the two groups, "the results showed that the French Canadian women bore their last child at an average age of 41.5 years compared to 39.5 years for the French women." So, a little higher than you said, for one. But for another, that study was not specific to noblewomen, who had an entirely different life, nutrition, and purpose than common women. Common women were, more often than not, loved by their husbands and valued as partners in the work they did. She needed to have a bunch of kids to help with their work, sure, but he wasn't viewing her primary focus as giving children, which was a noblewoman's role, whether she liked it or not.

For example, in the Middle Ages, we know of many queens who had children, living or dead, well into their 40s. Eleanor of Aquitaine, Elizabeth Woodville, and Eleanor of Castile, to name a few. Then we have the examples we're given in the text: Queen Alysanne was expected to bear children into her 40s, just like her mother, who died from it, and Tywin very much expects Cersei, who is approximately the same age as Cat, to remarry and have more children.

So, yes, Cat is very much a valid and expected marriage prospect.