She's the younger one. I think it seems to be one of those "just biology" things, whether a man consciously still wants children or not the fact is that if they start dating a younger woman that remains a possibility, as men don't lose their ability to father children until very old, while with us women the peak fertility is just around 20, and starts to decline after 30, and by 40 it's already gone enough that you are more likely to need some medical system help if you still want to conceive. So it's presumably not so much the outer beauty of younger women, it's what that more youthful look is advertising that attracts men - whether they realize it or not, and that works on them even when they don't consciously want children - while for women a bit older, or sometimes even a lot older man is often enough just fine as long as he seems like a good provider, and somebody who'd be able to take care of her children if they have them.

What can't breed and produce the next generation will die out, and no matter how much that may be ignored in our current culture it's still one of the most fundamental biological urges after the survival of the individual him- or herself. Even if we mostly just notice that urge to breed, the preferences who we want to do that with are still pretty strongly connected to who might be able to produce those children with us. Women who look still potentially fertile are seen as more attractive, with men is mostly the ones who look like they could feed and/or protect those children and their mother if she has them, especially when they are young.