At what age do children become self conscious of how they look?

Firstly, I must preface that this has nothing to do with the particular parent, child or their parenting skills.  It could have been any child even my own.

We were at a Christmas party on the weekend.  There was a pretty little 4-year old girl in a lovely party dress.  She accidentally spilled a glass of red wine all over her dress.  Her mother whisked her off to change, but she was very upset, not because she was worried about being told off, as it was a genuine accident, but because she ‘didn’t look pretty anymore’.  This comment freaked both her mother and I out a bit and we did chat about it.  The mother also mentioned that she overheard the girls use the word ‘skinny’ in the nursery playground but didn’t know in what context.

This isn’t the first time.  When my daughter was two I distinctly remember her pal leaning over to her and saying ‘I like your jeans’.  I was absolutely stunned. WTF?  How does a 2-year-old know what they like and what they don’t like when it comes to fashion.  I can appreciate that they may like chocolate, toast or mummy and daddy but clothes??  They couldn’t even dress themselves yet.

How does this happen at such a young age?  Is it parental role models? Media?  How can we stop it from happening?

I would love to hear your ideas.