Ah...and this is where some anthropology comes in, with a smattering of ancient history. There have been cultures with wildly different takes on things-there is a lot of evidence that some prehistoric cultures were matriarchal, with age and wisdom being prized as much as childbearing-and the same goes for some tribal cultures recently. Though at first sight it would make sense to think that once a woman was past childbearing age they would no longer have much respect, most matriarchal/semi matriarchal cultures seem to elevate older women into a position of being seen as teachers and sustainer's to the younger women and thus essential. While being youthful and fit is always a good thing for childbearing, this is just one role among many and the sustainer/teacher role is also prized in some cultures-a woman just moves from one role to another. So-yes its generally preferable for having kids-that makes sense-but its not always been the most elevated role a woman can have.

The image of beauty is strictly cultural as well-in many countries big is seen as beautiful because it is evidence the person is rich enough to eat well. You also get practices such as scarification and lip plates being seen as enhancing beauty.

There is also a tribe in Africa where the men nurse the children while the woman do important work-they take the baby with them through out the day, letting it suck on their nipples as a 'dummy', and its not thought in any way un-macho.

We just rarely if ever get to see evidence of these different lifestyles because, like most cultures, we tend only to see the things that fit with and encourage our own worldviews.