Magazines like Seventeen, YM, and Jane, while stating they are aimed for older teenagers, are actually purchased by younger girls 16 and under. They are heavy with advertisements, with the majority following the trend that sex sells. Girls are groomed to buy, buy, buy while emulating those that are popular, beautiful, and of course, accepted. The more designer outfits and the 'right' makeup, the more sought-after the owner is. And isn't this the same as the real couture magazines? In order to be sucessful, you must dress (and look) the part. Who can resist you if you've got all that?
Magazines that are geared to younger audiences are merely stepping stones for the 'real' thing. Sex sells, plain and simple, to those that are active participants, and those that know that one day, they're gonna be.