Its just a societal value, other societies in the past viewed this as a sign of youth and nothing more.
In Japan, the phenomena is called kawaii (or cute) and focuses on the social rejection of the dominant working class concept of the adult. In the same way, these could be read as a focus on the rejection of age and the clinging towards this youth as a primary concern in our society.
As for the sexy factor, the only reason we view this as wrong is because we associate youth with innocence and purity, but again this is a social construct. Perhaps it can be viewed as a rejection of this ideal and a furthering of the feminist ideal of breaking the shackles at an earlier age, who knows.
As for the young male models, it can be offset by the metrosexual concept, whereby there is a gender clash, and I have seen a few advertisements of certain younger looking male models in gender-bending situations or otherwise 'unexpected' from the traditional model of the masculine.
In the end, its all a crock of shit, and I suspect there are far more important matters to kick up a storm about rather than debate on the movements and concepts behind fashion shoots.