I think the key word in the post title is "extremely." Models have always been young and significantly younger than the buyers of the products they represented so that is nothing new. I think there is a difference between having a model who is young fronting a campaign and a model who looks young. I remember coming across an interview that I think was from the supermodel era give or take a few years - a comment was made that designers (and presumably editors) want models who are seventeen chronologically but who can look twenty-seven after she has been styled and glammed, I think that is the longer-term view of models.
However, that particular view of models was turned upside down when the baby doll look came into vogue, then a model who was sixteen and looked sixteen became in demand and that made sense when the clothes and the editorial themes aligned with that look. Now that the baby doll looks and themes are passe I think that it is taking the fashion world time to adjust its thinking to the new normal which is really the old normal. Also, I think that there is a lot of noise in the system, like it is not unprecedented for a very young model (15-18 range) to be an It Girl and score the high profile gigs, but they were the exception and not the rule. Nowadays, it's like "well Jac and Karlie are two of the hottest models and we have to fill our boards, runways and pages with models of that same profile." I think that today agents associate freshness and discovering a new face with plucking a girl out of the mall or off the tundra and immediately putting her on a plane to Milan. Correct me if I am wrong, but I think that the Joan Smalls situation is more representative of how it used to happen. Yeah she may have started modeling as a teenager but she made her bones doing catalogs, and low profile shows and print work, so when she was cast in the Givenchy show it was still considered as a discovery of a fresh new face. To me Joan Smalls story more closely parallels how models were discovered in previous eras (again correct me if I am wrong), it's not this 0 to Milan in sixty seconds thing that we have going on now for entire groups of models.
Getting back to the original question, while I agree that most industrialized cultures are youth obsessed, I just can't bring myself to believe that women in their thirties and forties want to look like they are seventeen, twenty-seven yes, but not seventeen. When I think high profile campaigns from recent seasons, I don't think that they were littered with models who looked like they are fifteen - to me Jac did not look fifteen in the Calvin Klein ads, Karlie doesn't look seventeen in the Dior ads. However, Karlie looked sixteen-ish in the Hermes campaign and even though her face was mostly obscured, Rasa definitely looked like a young 'un in the SS2010 Prada ads, I mean the pigtails were a pretty big clue. Still in the Hermes and Prada situations, the model's youth fit the theme. Another element of noise with this issue is that brands like DKNY, Blu-Girl, Miu Miu and Marc by Marc Jacobs target people in their teens and twenties so a youthful theme and cast makes sense.
I think the real mistake being made by designers and photographers in thoughtlessly looking to the pool of fresh, usually teen, faces for campaign casting is not necessarily looking the part physically but it is more about these newbies' skills as models and their presence. Does a model who is still in high school have the drawing power to make someone stop flipping through the magazine to examine an ad? Most likely no, IMO. Now I will throw in a caveat, sometimes the styling, set up and photography is enough to draw you in and model skill and presence or lack thereof is not a barrier to the campaign being effective, Prada SS2010 with Rasa is an example of this. While I liked the Prada SS2010 collection, I question why a designer who has a youth brand (Miu Miu) would even choose to slot those designs in her mainline. As I said previously the model casting made sense in that case, but it should be noted that the following season Miuccia was extolling the virtues of grown women.