For my purposes it is most significant that youthfulness was a key element of this ubiquitous ideal of feminine beauty. The ages of some Middle English literary heroines who are described as beautiful are stated, and they are always in their teens. Freine is twelve when men begin to notice her beauty, and Goldboru, 'the fairest wuman on live' is an heiress waiting to come of age. Chaucer's Virginia -- a 'mayde of excellent beauty' who is described in conventional terms -- is fourteen. Where the age of the beautiful woman is not stated, her youth is implicit in her figure, with its slender limbs, small high breasts, narrow waist and smooth white skin. An ugly woman, in contrast, is often old, with loose skin, a forest of wrinkles and breasts like deflated bladders.
The author is a post-teenage female, yet in her chapter there is none of the bitterness or scorn that we'd expect from Sailer's rule of female journalism. Aside from the age of girls in literature, she discusses the era's obsession with the Coronation of the Virgin. When Mary is crowned in Heaven, she argues, she should be shown in the idealized female form, just as Jesus is shown in the idealized male form (he usually looks like he's in his late 20s to mid 30s). The key detail is that usually Mary does not wear a wimple, which only married women wore (the average age at marriage for females was early to mid 20s). Rather, she's shown with long, luxurious flowing locks, as well as immaculate and fair skin and youthful facial geometry. She never looks 12, but mid-to-late teens, sure. (When Goldboru is described as "an heiress waiting to come of age," remember that girls went through puberty much later before the 20th C, probably around 17 instead of 13.)
Looking through the various paintings at the Wikipedia page and through Google Images, it's clear that this pattern isn't always there. Most of the Italian and Spanish paintings show her with a wimple and sometimes a matronly face, while the Northern European ones usually show her as youthful and with uncovered hair. At least in Chaucer's neck of the woods, the overall story seems to hold up. The only half-baked idea I have for the North-South difference is that in Northern Europe the family structure was more nuclear, while the Southern European structure was more extended. When your view of marriage is finding the right one and running off to start your own household, you'll idealize your high school sweetheart more than if your view of marriage includes a matriarch ruling over a mini-empire of a household.
While the ideal of beauty is not the same at every time and in every place, it's clearly bounded. In no society is the old maid chased after as she is among some of our primate relatives. The contemporary American ideal must be pushing the upper limit, what with post-menopausal women appearing on the cover of Playboy and the greater focus on housewives than sorority girls. These are the tails of the idealized age distribution, but still -- you wouldn't have seen that in most other places and times. Northwestern Europe during the Late Middle Ages is a case from the lower bound. And we're not talking about a greasy heavy metal band singing about jailbait -- the examples come from high literature and religious paintings.
No social disapprobation for finding teenagers attractive, plus a widespread low-carb diet? I was born in the wrong century. Yeah, there were all those disasters that upended the 14th C, but I would've made it through. And with a lot of the land cleared out, the survivors must have had a relatively comfortable standard of living, enabling them to think about higher concerns than "where's the food at?"