In a young girl's mind she's a beautiful princess. In her tweens, she's a model and utterly gorgeous - she's going to conquer the world. By mid teens, her world starts to change, she sees imperfections ... it's the slippery slope. This is a generalisation but I'm sure you get the picture.
Girls are becoming younger and younger when they start to focus, or should I say, obsess about their appearance. Most carry this through into adulthood. Some grow from the experience and see it for what it is, some hold on to this baggage. And it doesn't matter how attractive they are, if there's even the slightest imperfection they'll hone in on it.
WHY DO WE CHANGE?
There are many reasons why we change but a lot can be attributed to external influencers such as celebrity beauties who are impossible to measure up to. Try as hard as they may, teenagers cannot reach those dizzying heights - they don't have the money, the professionals (makeup artists, hair stylists, personal trainers, etc) or the likes of Photoshop to perfect their look.
Just about every woman I meet has some kind of hang up about themselves and/or their body. That becomes their focus. Never mind that 80-90 per cent of them is beautiful and amazing, they focus on the 10-20 per cent that's not.